1. Soak fillets in lemon juice for 15 minutes and rinse.
2. Sauté diced onions in oil until golden brown. Add ketchup and spices, mix well. Remove from flame.
3. Place sliced potatoes on bottom of nine- by 13-inch pan and lay fillets on top. Pour sauce over fish and cover well.
4. Bake at 350 degrees Fahrenheit for one hour and 15 minutes or until potatoes and fish are soft.