Combine first 3 ingredients in a large heavy-duty, zip-top plastic bag. Add bread cubes; seal bag, and shake until cubes are well coated. Arrange bread cubes in a single layer on a 15- x 10- x 1-inch jellyroll pan coated with cooking spray. Bake at 350° for 15 minutes or until golden, stirring occasionally. Remove from oven, and let cool completely.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion, sugar, and paprika; saute 10 to 12 minutes or until onion is tender and lightly browned. Transfer onion mixture to an 11- x 7- x 1 1/2-inch baking dish coated with cooking spray. Place fish over onion, and sprinkle with lemon juice, salt, and red pepper.
Wipe skillet dry with a paper towel. Coat skillet with cooking spray; place over medium-high heat until hot. Add green pepper and mushrooms, and saute until tender. Stir in tomato. Spoon mushroom mixture over fish; drizzle with sherry. Cover and bake at 400° for 15 minutes. Uncover; top with bread cubes. Bake, uncovered, 5 additional minutes or until fish flakes easily when tested with a fork.