Preheat oven to 350°.
Wrap buns in foil; bake at 350° for 10 minutes or until thoroughly heated. Remove from oven, and keep warm.
Bring 2 quarts water to a simmer in a large saucepan. Add franks; simmer 5 minutes or until thoroughly heated. Drain well. Place 1 frank in each heated bun. Top each frank with about 2 tablespoons onion, about 2 tablespoons tomato, about 1 tablespoon relish, and about 2 teaspoons mustard. Serve with peppers.