Summer Squash Casserole

4 lb. yellow summer squash, sliced
4 Tbsp. butter, divided
1 large sweet onion, finely chopped
2 garlic cloves, pressed or finely minced
2 1/2 C. soft breadcrumbs, divided
1 1/4 C. shredded Parmesan cheese, divided
4 oz. shredded sharp Cheddar cheese
1/4 C. chopped fresh chives
1/2 C. minced fresh parsley
8 oz. container sour cream
2-3 tsp. salt
1 tsp. freshly ground pepper
2 large eggs, lightly beaten
1/4 tsp. garlic salt

Cook squash in boiling water to cover in a Dutch oven 8-10 min. or just until tender. Drain well; gently press between paper towels.
Melt 2 Tbsp. butter in Dutch oven over medium-high heat; add onion and garlic, and saute 5-6 min. or until tender. Remove from heat and stir in squash, 1 C. breadcrumbs, 3/4 C. Parmesan, and next 7 ingredients. Spoon into lightly greased 9 X 13 in. baking dish.
Melt remaining 2 Tbsp. butter. Stir together melted butter, remaining 1 1/2 C. breadcrumbs, 1/2 C. Parmesan cheese, and garlic salt. Sprinkle mixture evenly over casserole.
Bake at 350 for 35-40 min. or until set.