Begin by heating a large saucepan and sauté the green pepper and garlic in the margarine until the pepper is tender. Stirring until smooth, add the flour and then continue to cook for 1 minute, stirring constantly.
Cooking over medium heat, gradually add the milk and seasonings to the saucepan. Stirring constantly, cook until the mixture is thickened and bubbly. Stir in the onions, tomatoes, and corn, and remove from the heat.
Preheat your oven to 350 degrees F and spoon the mixture into a 2-quart casserole dish coated with cooking spray.