Preheat oven to 375°. Unroll crescent dough and separate into 2 rectangles; press perforations to seal.
Spread rectangles with mustard. Top with cheese, roast beef, overlapping if needed, and if desired, red pepper. Roll up jelly-roll style, starting with a short side; pinch seam to seal. Cut each roll crosswise into 4 slices; place on parchment-lined baking sheets, cut side down. Bake until golden brown and cheese is melted, 15-18 minutes. Serve warm.
Freeze option: Cover and freeze cooled pinwheels on parchment-lined baking sheets until firm. Transfer to freezer containers; return to freezer. To use, bake at 375° until heated through.