Layer the sliced yellow and zucchini squash in the bottom of an 8 quart crock pot. Add the bell pepper, onion, celery and garlic. Toss the vegetables together. Toss melted butter with the stuffing mix and fluff with a fork. Set aside once cup of stuffing mix for later.
Add the canned cream soup, shredded cheese, salt and pepper. Stir with a large wooden spoon to blend the soup, cheese and seasoning with the vegetables. Add the stuffing mix to the crock pot and stir well to evenly combine all of the ingredients.
Save the remaining cup to used as a topping before serving. Place the lid on the slow cooker. Slow cook on high for 3 hours or low for 4 to 5 hours or until the yellow squash and zucchini slices are cooked tender.
Sprinkle the top of the casserole with the remaining one cup of stuffing mix and serve it right from the slow cooker. Or you can do as I have done and transfer the hot casserole to a casserole baking dish for a prettier presentation.