In a large skillet, cook green peppers and onion in oil over medium-high heat until tender, 8-10 minutes. Without separating rolls, cut each package in half horizontally; arrange bottom halves in a greased 13x9-in. baking pan. Layer with roast beef, pepper mixture and cheese; replace top halves of rolls. , In a small saucepan, melt butter; add dried onion, Worcestershire sauce and garlic powder. Drizzle over rolls. Cover and refrigerate 8 hours or overnight., Preheat oven to 350°. Remove rolls from refrigerator 30 minutes before baking. Bake, uncovered, 15 minutes. Cover with foil; bake until cheese is melted, 10 minutes longer.