Set the steaks or fillets in one layer in an 8 X 12-inch (20 x 30 cm) glass dish Pour the liquid over the fish.
Cover with the onion and lemon slices, the bay leaf, salt and peppercorns.
Microwave, uncovered, 3 minutes at HIGH.
Tilt the dish and pour the cooking juice over the fish with a spoon.
Microwave 4 minutes more at HIGH.
Cover and let stand 3 minutes.
Remove fish to a warm platter with a perforated spoon.
Mix together the butter and flour, add to the fish juices, mix thoroughly.
Add the lemon juice, parsley or dill.
Microwave 2 minutes at HIGH.
The sauce should be light and creamy; microwave 1 minute more, if necessary.
Pour over the fish.