That is a whole bunch of onions!! But it's a good cause. The only onion casserole recipe I have in my file is called "Onion Pie" from DonnaOH.

I freeze onions but only use them in soups or stews. I just dice them and put them in freezer bags. They work fine if you're cooking with them.

Here's the recipe. May not be what you're looking for but it sure sounds yummy.

Onion Pie

1 c. crushed saltines (or 20 Ritz crackers)
5 Tbsp. butter, melted
2-1/2 c. onions thinly sliced (about 2 medium)
2 Tbsp. oil
2 eggs
3/4 c. milk
1/4 c. shredded cheese (more if needed)

Preheat oven to 350. Combine cracker crumbs and butter and press into a pie pan. Bake 8 minutes (glass takes a little longer). Saute onions in oil until tender--place in cracker crust shell. Mix remaining ingredients (EXCEPT cheese). Pour over onions. Top with cheese and bake 45 minutes.

I chop onions and freeze them. Lately I've made up a "trinity", onions, celery and bell pepper chopped up and froze in bags or you could use containers. If not sealed well, you need to use quickly though.

That's the one I have, is Donna's and she fixed it for me, once when I visited her and love loved loved it.

This is Paula Deen's onion pie.

****only thing I would do differently is cut the rings in half at least for easier eating.

3 cups thinly sliced Vidalia onions
3 tablespoons butter, melted
1 (9-inch) prebaked deep-dish pie shell
2 eggs, beaten
3 tablespoons all-purpose flour
1/2 cup milk
1 1/2 cups sour cream
1 teaspoon kosher salt
4 slices bacon, crisply cooked and crumbled
Directions
Preheat oven to 325 degrees F.

In a medium saucepan, melt the butter over medium heat. Add the onions and saute until lightly browned.

Put the pie pan on a sheet pan. Line the bottom of pie crust with the onions.

In a small bowl, beat the eggs and the flour together to combine. Add the milk, sour cream and salt. Mix well and pour over the onions. Garnish with the bacon and bake until firm in the center, about 30 minutes. Transfer to a serving platter and serve.

This is an onion recipe that I've made numerous times. It is very good. In fact, we liked it so much we made it several times within just a few days!

ONION BAKE

6 onions, thinly sliced
2 cups butter
1 (10.75 ounce) can condensed cream of chicken soup
1 cup milk
2 cups shredded mozzarella cheese
1 (1 pound) loaf French bread

Preheat oven to 350 degrees F (175 degrees C).
In a small pan, melt butter and add onions. Cook 10 to 15 minutes or until soft. Remove onions from pan (leave butter in pan) and place in a 9x13 inch pan.
Combine soup and milk to pour over onions. Sprinkle with cheese. Slice bread approximately 1 1/2 inches thick and dip both sides in butter. Arrange on top of cheese. Bake for 30 minutes or until bread is golden brown and toasty!