Combine the garlic, rosemary, salt and pepper; set aside 4 teaspoons. Rub the remaining mixture over roast; place in a greased shallow roasting pan.
In a small bowl, whisk reserved herb mixture with oil. In a large resealable plastic bag, combine the potatoes, yellow peppers and onion; add oil mixture. Seal bag and toss to coat. Arrange vegetables around roast.
Bake, uncovered, at 425° for 30-60 minutes or until meat reaches desired doneness (for medium-rare, a meat thermometer should read 145°; medium, 160°; well-done, 170°).
Transfer roast and peppers to a warm serving platter. Let stand for 10-15 minutes before slicing. Meanwhile, return potatoes and onion to the oven; bake 10 minutes longer or until potatoes are tender. Yield: 6 servings.