Heat oven to 450 degrees. Spray a 13x9−inch baking dish with nonstick vegetable cooking spray. Arrange fillets, slightly overlapping, in the prepared dish. Set aside.
In a medium bowl, combine carrots, melted margarine, juice, thyme, and salt. Spread mixture evenly over fillets. Cover with foil. Bake for 25 to 30 minutes, or until fish is firm and opaque and just begins to flake. Drain liquid from fish into a 2−cup measure. Cover fish with foil to keep warm. Set aside. Add water to liquid in cup to equal 1 1/3 cups.
In a 1−quart saucepan, melt 3 tablespoons margarine over medium heat. Stir in flour, salt, and pepper. Blend in cooking liquid mixture and milk. Cook for 5 to 7 minutes or until mixture thickens and bubbles, stirring constantly. Pour sauce evenly over fish and serve over hot rice or linguine.