Oven takes 15 minutes at 350 degrees. Open the foil and cover with the shredded cheese, put it back over heat uncovered. In a small bowl, mix fish, cucumber, radish, mayonnaise, sour cream, salt and pepper. Place one tomato slice on each of four toasted bread slices. Top each piece with one piece of cheese.
Place sandwiches on baking sheet.
Broil 2 to 3 inches from heat until cheese melts, about 2 minutes. Top each sandwich with another piece of toast, if desired. 2 to 4 servings. Wash and pat dry with paper towels. Grill the fish for 3 to 4 minutes on a side, depending on thickness, turning gently.