Place a large frying pan, with a heavy bottom on low heat. Melt butter or margarine in the pan. Sauté the onion over moderately low heat and add the minced garlic. Make sure to stir often. Stir in the flour, making a roux while stirring, for about 3-5 minutes. Pour in the milk in a slow, steady stream. As you add the milk, use a whisk to vigorously stir until the mixture is thick and smooth. If you want the tigres to be spicy, stir in the 1/2 of a small pepper now. (If using pepper flakes, you can wait.) Add salt and pepper to taste. Simmer the béchamel sauce for about 10 minutes. Stir in the chopped mussels and the tomato sauce. Remove from heat.
If you are using pepper flakes, stir them into the mixture now.
Be sure that the inside of the mussel shells are clean.
