Heat oven to 450 degrees F. Coat a baking pan just large enough to hold the fish with nonstick cooking spray.
In medium-size bowl, gently stir together white wine, capers, mint and tomatoes.
Season fish with salt and pepper. Place in prepared baking dish. Pour tomato mixture over fish. Place a lemon slice on each fillet.
Bake in 450 degree F oven until fish is cooked through and flakes easily, 17 to 18 minutes. Remove fish to warm dish. Whisk butter into pan juices.
While fish is cooking, prepare couscous following package directions, using the butter, salt and pepper.
To serve, divide couscous among 4 plates. Spoon sauce from pan over couscous and serve with fish.