Preheat oven to 350°.
. Combine milk and cheese in a blender or food processor; process until smooth. Pour into a large bowl. Stir in egg substitute and next 3 ingredients. Add bread cubes; toss gently to coat. Let stand 30 minutes.
. While bread mixture stands, heat oil in a large nonstick skillet over medium heat. Add onion; sauté 25 minutes or until soft and lightly browned. Stir into bread mixture.
. Spoon bread mixture into an 11 x 7-inch baking dish coated with cooking spray. Bake at 350° for 42 minutes or until set. Let stand for 5 minutes before serving.