In 12x8-inch dish, combine onion, butter and garlic.
Cook, covered with plastic wrap at HIGH 3 to 3 1/2 minutes.
Stir in tomatoes, mushrooms, wine and basil.
Cook, covered, at HIGH 3 minutes and at MEDIUM 3 to 4 minutes.
Meanwhile, season fish with salt, skin side only.
Roll up (skin-side in) and arrange seam-side down in sauce; spoon sauce over fish.
Cook, covered, at HIGH 5 to 6 minutes, or until fish is done.
Let stand, covered, 5 minutes before serving.