Cut each fillet in half lengthwise (down seam in the middle). Set aside.
For filling: combine sauteed onions, bread crumbs, salt and pepper. Drop filling into fillets, roll, and secure with toothpick through the center of the rolled fillet. Place fillets in pan.
Combine tomato sauce, water, lemon juice, and sugar. Pour over rolls. Bake uncovered for one hour.