Bring squash and water to cover to a boil; cook 30 minutes or until tender. Remove from heat; drain. Mash until smooth.
Melt 1 tablespoon butter in skillet. Add onion and garlic; saute 5 minutes or until tender. Remove from heat; stir in squash, 2 tablespoons butter, eggs, and next 3 ingredients. Fold in cranberries. Spoon into lightly greased 1-quart baking dish.
Combine breadcrumbs and remaining 1 tablespoon butter. Sprinkle over casserole.
Bake at 375° for 1 hour and 15 minutes or until set and golden.