In a very large skillet cook bacon over medium heat until crisp. Remove bacon to drain on paper towels, reserving 1 tablespoon drippings in skillet. Gradually add spinach to skillet, stirring frequently with metal tongs. Cook for 2 to 3 minutes or until spinach is just wilted. Remove spinach from skillet to a colander; hold over sink and press lightly to drain. (If using large leaf spinach, use kitchen scissors to snip.)
In the same skillet melt butter over medium heat; stir in mustard and crushed red pepper. Add drained spinach; toss to coat and reheat spinach if necessary. Top with cooked bacon. Serve immediately. Makes 4 to 6 servings.