In a large skillet, heat oil over high heat. Add bell peppers, salt, and black pepper. Saute' 12 to 15 minutes, or until tender, stirring occasionally. Transfer to a bowl, cover to keep warm. In the same skillet, heat beef broth over medium heat until hot. Add the thin roast beef slices and simmer 1 to 2 minutes, stirring gently. Spoon some broth onto the split hoagie rolls to moisten then evenly distribute sliced beef over rolls. Top with sauteed bell peppers and giardiniera, spoon on additional broth. Serve hot or warm, with plenty of napkins.