. An abundance of yellow squash and leftover ham led to this quick and easy supper dish. . Squash & Ham Casserole . 2 cups yellow squash 1 cup chopped ham 1 medium yellow onion 1 can cream of celery soup 1 cup cheddar cheese 1/4 teaspoon black pepper 1/4 teaspoon sea salt 1/4 teaspoon garlic powder . Pre-heat oven to 350 degrees. Wash and slice squash. Slice onion. Cover squash & onion with water in a medium pan. Boil for 15 minutes and drain. Add all ingredients to squash & onion. Pour into small casserole dish. Bake at 350 for 30-35 minutes.