In a small saucepan, heat the margarine until hot. Sprinkle with flour, and combine. Cook over low heat, stirring constantly, for 1 minute. Remove from heat; stir in water, wine, and mustard. Return to heat, and bring mixture to a boil; reduce heat and let simmer, stirring constantly, until sauce is thickened. Stir in 1 T lemon juice. Remove from heat and keep warm.
Sprinkle Tilapia fillets with remaining lemon juice. Spray broiling pan rack with cooking spray. Transfer fish to rack and broil until the fish flakes easily; approximately 5 minutes. Sprinkle fillets with salt, than spoon sauce over fish.