Heat oven to 450°F. Spread potatoes in single layer on an ungreased 18x13-inch half-sheet pan or large cookie sheet. Bake as directed on package.
Meanwhile, in 12-inch skillet, heat oil over medium-high heat. Add onion, bell peppers, salt and pepper; cook 10 minutes, stirring occasionally, until vegetables are golden brown and tender.
Microwave beef roast as directed on package. Cool slightly; shred beef, reserving 1/4 cup au jus. (Discard remaining juices.)
Scatter vegetable mixture evenly over fries on pan. Top with shredded beef. Drizzle with reserved au jus. Sprinkle with cheese.
Bake 3 to 4 minutes or until cheese is melted. Serve immediately.