In a very large skillet, cook bacon over medium heat until crisp. Drain bacon on paper towels, reserving 1 tablespoon drippings in skillet.
Use kitchen scissors to snip any large spinach leaves. Gradually add spinach to skillet, stirring frequently with metal tongs. Cook for 2 to 3 minutes or just until spinach is wilted. Transfer spinach to a colander; hold over sink and press lightly to drain.
In the same skillet, melt butter over medium heat. Stir in mustard and crushed red pepper. Return spinach; toss gently to coat. If necessary, reheat spinach. Sprinkle with cooked bacon.