Sprinkle salmon fillets with salt and pepper. Place the fillets in a large skillet or small roasting pan. Spoon chopped tomato around fillets, and drizzle fillets with soy sauce. Wrap handle of skillet with foil, and bake the fillets at 425° for 12 minutes or until fish flakes easily when tested with a fork. Remove fillets form pan, and keep warm. Add the chopped tarragon and mustard to tomato, stirring well. Spoon tomato mixture over fillets.