In saucepan mix squash, garlic, onion, salt and water. Bring to simmer; cover and cook, adding small amounts of water if needed as liquid cooks away, until squash is tender, 20 to 30 minutes.
Remove from heat. Drain well and mash hot squash mixture with 2 Tbsp. of butter.
Beat together evaporated milk, eggs and beat into squash. Add all but 3 Tbsp. of cheese and enough of cracker crumbs (1/4 cup) to thicken mixture. Combine remaining cracker crumbs with 2 Tbsp. of butter and reserved 3 Tbsp. of cheese. Sprinkle on top of casserole and bake until bubbly and browned, 30 to 35 minutes.
