I cook these often, and they are quick. Place a bit of water (1/8th") in the bottom of a non-stick pan. Place the mushrooms in the pan with the undersides facing up. Cut little pieces of butter and evenly drop them on the mushrooms. Place some peeled garlic cloves in the pan and on the mushrooms. Cover and simmer on low heat. About 30 minutes later (or longer if you like them very soft) they make a good side dish or, when cool, make good sandwiches.

If you uncover the pan towards the end of the cooking process, the garlic will caramelize. We like the caramelized garlic as a condiment.
-hald

Sara Moulton just made these yesterday on her cooking show. She marinated the mushrooms in a mix of olive oil, fresh thyme sprigs, and balsamic vinegar and salt and pepper, then sealed in a ziplock bag to marinate. I'd add some chopped parsley and a scallion.

Sara grilled hers indoors on a grill pan. You could broil them or sautee them indoors. Sara also removed the gills, which is a trick I'm going to try. I've seen her make both a grilled portabella sandwich and stuffed portabellas, in which case she stuffed them with a mix of toasted bread crumbs, onions, mushroom stems and I think green beans, topped with smoked gouda or something like that. She topped the grilled sandwich ones with gorgonzola and served them on grilled country bread brushed with olive oil and a clove of garlic. Also with chopped roasted red peppers.

I don't have a grill, so I broil things instead of grilling, or pan sear.

Here's Sara's Recipe I found online. Also a link to her Web site with the other recipe.

Portobello Burgers with Red Pepper and Gorgonzola
Makes 4 servings
Hands-on time: 20 minutes
Total preparation time: 30 minutes plus 1 hour marinating

This is an absolutely wonderful meatless summer sandwich. In the center is a marinated and grilled Portobello mushroom cap. It is filled with melted Gorgonzola, topped with roasted red peppers, and lathered with just a little mayonnaise, the whole of it pinned down between slices of toasted garlic bread. What could be better? Try adding this item to the menu the next tme you're grilling up hot dogs and hambies in the back yard, and I bet you'll win some converts.Of course, using your trusty grill pan or broiler, you can also enjoy this recipe in the dead of winter. Serve it open faced and cut into fourths as an appetizer at a dinner party If you don't love blue cheese, substitute any strongly flavored melting cheese. Whenever and however you make it, no one's going to miss the beef.

4 large Portobello mushrooms, stems removed
1/4 cup balsamic vinegar
3 garlic cloves, minced, plus 1 whole clove, peeled and halved
1 tablespoon chopped fresh thyme
1/4 cup extra virgin olive oil, plus more for brushing
2 small red bell peppers
Kosher salt and freshly ground black pepper to taste
1/2 cup crumbled Gorgonzola cheese
Eight 1/2-inch-thick slices rustic country bread, cut from a 6-inch-high loaf
1/2 cup mayonnaise
1/2 cup shredded fresh basil

Wipe the mushroom caps clean and use a small spoon to scrape out the gills on the underside. Place 3 tablespoons of the vinegar in a small bowl and whisk in the minced garlic, thyme, and 1/4 cup olive oil. Divide the mixture between 2 large plastic bags with resealable closures. Put 2 mushroom caps in each bag and seal, pressing out the excess air. Marinate the mushrooms at room temperature for at least 1 hour and up to 2 hours, turning the bags often.

Prepare a charcoal fire and let the coals burn down to a gray ash. Lightly oil the grill. Set the peppers on top and grill, turning often, until the skins are blackened on all sides, 10 to15 minutes. (Alternatively, preheat the broiler to high and place the broiling pan 4 to 5 inches from the source of heat. Set the peppers on the pan and broil, turning often, until blackened on all sides, 12 to 15 minutes.) Place the charred peppers in a large bowl and let stand, covered with plastic wrap, until cool enough to handle. Peel, core, and seed the peppers and then cut into thin strips. Toss the strips with salt and pepper.

Remove the mushrooms from the marinade, pat dry, and season with salt and pepper. Place the mushrooms cap sides up on the grill and cook until dark and slightly softened, 2 to 3 minutes. Turn and grill the other side until tender, 2 to 3 minutes longer. (Alternatively, the mushrooms can be grilled in a hot, well-seasoned ridged grill pan over medium-high heat for about 5 minutes per side, but grill the bread first; see below.) When the mushrooms are almost tender, sprinkle a quarter of the cheese into the cavity of each cap.

While the mushrooms are grilling, brush the bread with olive oil on both sides. Grill until lightly toasted, turning often, 3 to 5 minutes. Rub one side of each slice with the cut garlic clove while hot. (If using a grill pan, grill the bread first and then the mushrooms.)

Add the reserved tablespoon of vinegar to the mayonnaise. Stir well and season with salt and pepper. Spread on the garlic-rubbed side of each slice of bread. Top half the bread slices with a mushroom cap, equal amounts of the peppers, and basil. Place the other bread slices on top. Cut the "burgers" in half and serve.

Here is a link that might be useful: Sara's Weeknight Meals - Vegetable Plate

Hey, I suppose this is years later, but I've often cooked Portabello Mushrooms in my toaster oven. I clean the mushroom, reserve the stems for another purpose (stocks or soup, usually) center it on a piece of foil big enough to fold over the mushroom, then drizzle on a little olive oil, a little balsamic vinegar, maybe soy sauce or a little salt. Sometimes I have added herb mixes, like Mrs. Dash or herbs de provence. Sometimes I put some crushed garlic in there, too. Then I fold over the foil, making a packet and roast the mushroom for about 45min - 1hr or so at 400. When you unwrap the mushroom, be careful, there will be a lot of hot juices in there too. They are delicious, don't lose them. After you pour the liquid off the mushroom, slice it up.I usually have my mushroom as a final topping over a pasta dish or rice. If rice, then I just go ahead and pour the liquid on there as an additional flavoring, the rice will absorb it. Or I capture the liquid in a jar and use it to flavor some other dish later, or freeze it as an ice cube to save.