Preheat oven to 325°.
Combine all ingredients except cooking spray in a large bowl. Spoon onion mixture into 6 (6-ounce) custard cups coated with cooking spray. Place the custard cups in a 13 x 9-inch baking pan; add hot water to pan to a depth of 1 inch. Bake at 325° for 1 hour or until a knife inserted in center comes out clean.