This is the recipe I use since it's most like the kind I grew up with. Not fancy but very good. You can use any other cheese you like:
STUFFED GREEN PEPPERS (serves 4)
2 large green peppers
1/2 lb lean ground beef
1/4 cup uncooked instant rice
8 oz can tomato sauce, divided
2 TBL chopped onion
1 egg, beaten
1/2 tsp salt
1 TBL grated Parmesan
1/2 tsp Worcestershire
Dash of pepper
Cut peppers in 1/2 lengthwise & remove seeds. Combine meat, rice, 1/4 cup tomato sauce & remaining ingredients. Stuff peppers with mix & place in a greased baking dish. Spoon remaining tomato sauce over stuffed peppers & then sprinkle w/additional Parmesan. Cover & bake in a preheated 350° oven for 50-60 minutes.~~ You can mix mozzarella or cheddar into the meat mixture as well. From By Special Request

I love stuffed peppers. We do them with the rice and burger recipe also but I try to do them Italian style once in a while. We brown spicy Italian sausage and drain. Put in a medium mixing bowl. Add a large can of diced tomatoes, a diced onion, small can of sliced olives, some diced peperoni, sliced mushrooms, and mozzarella cheese. I didnt give measurements cause I never really use a recipe. Just kinda put in there what I like on my pizza. I cover it and bake it at 350 for about 50 mins then remove the cover sprinkle mozzarella on top and put it back in under the broiler for a couple of mins. Make sure to drain the tomatoes, olives and mushrooms before adding and I always put the peperoni on a microwave safe plate between 2 paper towels for about 20 seconds to remove some of the grease before I put it in the mixture. How you cut and stuff the peppers is up to you. The kids love these!

Home Cookin Chapter: Recipes From Thibeault's Table

Meat Stuffed Peppers
====================

Lidia's

Meat Stuffed Peppers
Peperoni Imbottiti

Description:

Ingredients: 1/3 cup Arborio rice
Salt
8 cubanella or banana peppers, each about 6 inches long (green peppers work just as well)
3 tablespoons extra virgin olive oil
1 large yellow onion, chopped (about 1 cup)
8 ounces ground meat (see Note below)
1 egg
1/3 cup grated Parmigiano-Reggiano cheese
3 tablespoons chopped fresh Italian parsley
2 teaspoons chopped fresh oregano
3 cups (or as needed) Tomato Sauce or sauce that accompanies Chicken Parmigiana, New-Style

Directions: Peppers with a slight kick to them, like the cubanellas or banana peppers suggested above are wonderful for this dish. If you canÂt find those, choose the long, thin-skinned peppers often sold as ÂItalian frying peppers in supermarkets. You can serve the peppers alone or with a side of rigatoni, dressed with some of the sauce from the stuffed pepper-baking dish, grated Pecorino Romano cheese and a drizzle of olive oil.
Cook the Arborio rice in a large saucepan of boiling salted water until al denteÂtender but firmÂabout 12 minutes. Drain and cool to room temperature.
While the rice is cooking, preheat the oven to 400° F, prepare the peppers and start the filling: Cut the stems from the peppers and scrape out the seeds and membranes with a teaspoon. Heat 2 tablespoons of the olive oil in a small skillet over medium heat. Stir in the onions and cook, stirring, until wilted, about 4 minutes. Scrape the onions into a mixing bowl, add the ground meat, egg, grated cheese, parsley, oregano and cooked rice, and stir together until evenly blended.
Divide the filling among the peppers, using about ¼ cup to fill each pepper loosely. Rub the outside of the peppers lightly with the remaining tablespoon of olive oil, placing them into a 13 x 9-inch baking dish as you do so. Roast the peppers, turning once or twice with tongs, until softened and lightly browned in spots, about 20 minutes.
Pour in enough of the tomato sauce to barely cover the peppers. Cover the dish with aluminum foil and bake until the peppers are tender and the filling is cooked through, 30 to 40 minutes. Remove and let stand 10 minutes before serving.
Note: A mix of ground beef, pork and veal is best, but that might not be practical. Try at least to use a blend of ground pork and beef, but failing that, all ground beef or pork will do.

I used the tried and true Betty Crocker recipe for my husband, found i even liked it! And have used it ever since. I think parboiling the bell peppers was the key.

*** Stuffed Green Peppers (Betty Crocker)

4 large bell peppers, parboiled 5 min.

8 oz. ground beef
8 oz. tomato sauce
1 c. dry bread crumbs
1 T. minced onion
1 tsp. salt (scant)
1/4 tsp. pepper

Mix all ingredients and stuff peppers.
Stand in bread pan, or pan to fit.
Bake, covered, at 350 degrees for 45 min.
Uncover and bake another 15 min.