Begin by cutting the tops off of the tomatoes and scooping out the pulp, leaving the shells intact. Chop and reserve the pulp for later use. Invert the tomato shells and then let drain on paper towels for 10 minutes.
Omitting any salt or fat, cook the spinach according to the package directions. Drain well.
In a medium saucepan, melt the margarine over low heat. Add the flour, salt, mustard, and Worcestershire sauce, and stir until the mixture is smooth. Cook for 1 minute, stirring constantly, and then stir in the spinach and tomato pulp. Remove from heat.
Preheat your oven to 400 degrees F and then spoon the spinach mixture into the tomato shells. Top with the breadcrumbs and then place the stuffed tomatoes in a shallow baking dish treated with cooking spray.
Bake for 15 minutes, or until the mixture is heated well and the breadcrumbs and browned.