Preheat oven to 350°. Melt butter or margarine in a large skillet and sauté onions until tender. Add sour cream and mix thoroughly. Scoop half of onion mixture into a greased casserole dish. Top with 1/4 cup Parmesan cheese. Layer remaining onion mixture on top and sprinkle with remaining Parmesan cheese. Top with crushed Ritz crackers. Bake for 30 minutes.