If fish is frozen, thaw at room temperature until fillets will separate.
Rinse lightly and pat dry with paper towels.
Preheat oven to 350° and lightly grease a baking dish that will hold all the fillets in one layer.
Arrange fillets in dish, sprinkle with lemon juice, thyme, and salt and pepper to taste.
Bake for 25 minutes or until fish begins to look opaque.
Mix together the crumbs and cheese; remove dish from oven and sprinkle cheese mixture over the top evenly.
Dot with margarine and return to oven for another 10 minutes or until fish flakes easily with a fork.