I'm not 100% sure what you mean by a summer casserole - do you mean something with seasonal veggies such as zucchini and tomatoes? The only thing I could come up with right off the top of my head is veggie lasagna - this is a good recipe.

VEGETABLE LASAGNE

1 pound lasagne noodles, cooked and drained
1 cup (4 oz.) shredded fontina cheese
1 package (10 oz.) chopped spinach, thawed and drained
1 cup shredded carrots
1 cup shredded zucchini
3 cups part skim milk ricotta (divided)
1 egg
1 onion, chopped
2 tablespoons oil
2 tablespoons flour
1/4 teaspoon nutmeg
1 cup chicken broth
1/2 cup grated Parmesan cheese

Mix together fontina, veggies, 2-1/2 cups of the ricotta and the egg.
Place layer of noodles in 13 x 9 pan. Spread half of ricotta mixture. Repeat layers, ending with noodles on top.
Cook and stir onion in oil under tender. Stir in flour and nutmeg. Stir in chicken broth and 1/2 cup ricotta. Cook and stir until mixture comes to a boil. Spoon over lasagna. Sprinkle with Parmesan. Cover and bake at 350 for 45 minutes. Uncover and broil about 2-3 minutes or until lightly browned.

8 servings

I saw one with fresh squash, sausage, etc that looked really good.

It's not exactly a casserole but quick to make, especially if you cut everything except the tomatoes up the night before or earlier in the day and store in baggies in the fridge. I usually serve it with brown rice or mashed potatoes.

SUMMER SKILLET SUPPER

1 (1 lb.) fully-cooked ham slice
2 large tomatoes
1 clove garlic, minced
1 medium size green pepper
1 medium size onion
1 medium size yellow squash (about 1/2 lb.)
2 medium zucchini (about 1 lb. total)
3 Tbs. olive oil
1/2 cup chicken broth or 1 envelope instant chicken broth mixed with 1/2 cup water
2 tsp.fresh chopped basil or 1/2 tsp. dried basil
1/2 tsp salt, or to taste
1/8 tsp. pepper

Cut ham into 1/4 inch strips, about 1-1/2 inches long. Cut each tomato into 8 wedges; cut pepper into 1/2 inch wide strips and onion into thin lengthwise slivers. Cut squashes into 1/4 inch wide slices, halving lengthwise if larger than 1 inch in diameter.

In wok or large skillet over medium high heat, cook ham and all vegetables, except tomatoes, in hot oil about 5 minutes, stirring frequently. Reduce heat to medium. Add remaining ingredients, except tomatoes; stir and cook until vegetables are crisp-tender, about 10 minutes. Add tomato wedges and continue cooking until tomatoes are heated through.

This one gets raves everytime I make it. It's also great for making the night before and taking to someone the next day - reheats well.

Squash Casserole (with variations; I add baby eggplant,zucchini, swiss chard, green pepper, etc.)
3-4 medium yellow squash - quartered and sliced 1 inch thick
1 sweet onion - chopped
1 can mushroom soup
1/2 can evaporated milk
1 cup shredded cheddar cheese
1 egg
1 cup bread crumbs

Boil squash and onion (and other veggies if using) until tender. Drain and mash with a potato masher. Add soup, milk, 1/2 cheese, egg and 1/2 bread crumbs. Pour into greased casserole dish, cover with remaining cheese and bread crumbs. Bake at 350 for 30 minutes or until firm.

Enjoy!

Those recipies sound great. Thank you