Melt margarine in a pot and cook onions in it, 2 to 3 minutes or until translucent. Mix in flour and cook over medium-low heat, stirring continually for 1 minute. Pour in wine and milk and whisk until mixture boil. Continue boiling, whisking continually until thick, 1 minute.
Mix in cheeses, mustard, and Worcestershire sauce, cook over low-heat until cheeses are melted. Sprinkle with salt and pepper.
Coat both sides of bread with cooking spray. Toast bread over medium heat in a frying pan until golden, 2 to 3 minutes per side. Place tomato slices under a preheated broiler until warmed through. Lay bread on plates and top with tomatoes and cheese sauce. Garnish with parsley.