Cheesy Squash Casserole

1 tbsp vegetable oil
6 medium yellow summer squash, thinly sliced
1 large Vidalia onion, thinly sliced
1 tbsp butter
1/2 cup grated Parmesan
1 cup shredded sharp Cheddar
1/2 cup sour-cream
salt and freshly ground black pepper
1 sleeve Ritz crackers, crushed medium to fine

Preheat oven to 350. Grease a 2-quart casserole dish.
Heat oil in a large skillet over medium heat. Saute the squash, onion, and butter until soft.
Transfer to a bowl and stir in the parmesan, Cheddar, and sour cream. Add salt and pepper, to taste.
Place is the prepared casserole dish and sprinkle with the crackers crumbs evenly over the top.
Bake for 20 minutes or until the top is golden and bubbly.