1. Preheat oven to 400 degrees. Place peppers, onions, potatoes, and whole garlic cloves on a large rimmed baking sheet. Drizzle with half the oil, season with salt and pepper, and toss to coat.

2. Using a paring knife, make 12 small slits in top and sides of roast; push in garlic slivers. Move vegetables to sides of sheet. Place beef in center, and coat with remaining oil; rub all over with 1 1/2 teaspoons coarse salt, 1/2 teaspoon pepper, and thyme.

3. Roast 40 to 50 minutes, tossing vegetables occasionally, until tender and an instant-read thermometer inserted into thickest part of beef registers 130 degrees for medium-rare. Let meat stand 10 minutes, loosely tented with aluminum foil to keep warm. Cut half into very thin slices (reserve remaining half for leftovers). Serve with vegetables.

4. Cool remaining beef to room temperature; place in an airtight container, or wrap in plastic, and refrigerate.