I love using tiny potatoes and baby carrots, which makes this recipe insanely easy. That said, sometimes tiny potatoes cost a minor fortune. If that’s the case, save money by using larger boiling potatoes. You’ll have to spend time cutting them into cubes, but it might make sense.
You don’t have to use the nonstick foil here if you have a really good nonstick roasting pan, but it makes cleanup almost nonexistent, so I swear by it.
Preheat the oven to 450°F. Line a large roasting pan with nonstick aluminum foil. Add the carrots and potatoes. Drizzle 1 tablespoon plus 1 teaspoon of the olive oil over the vegetables and season them with salt and pepper. Toss just to combine, then distribute them in an even layer in the bottom of the pan, leaving room for the roast in the center.
Rub the roast all over with the remaining1 teaspoon olive oil, then season it with 1/4 teaspoon salt and 1/4 teaspoon pepper. Set it in the center of the veggies and pour the juice evenly over all. Sprinkle the thyme over the top. Roast the meat, carrots, and potatoes for 18 minutes. Then, stir the potatoes and carrots (if using nonstick foil, be careful not to tear it). Separate the onion quarters into pieces. Scatter them over the potatoes and carrots. Continue roasting until a meat thermometer inserted in the center of the roast reads 125°F; for medium rare, about 15 to 20 minutes more.
Carefully transfer the roast to a platter. Loosely tent the roast with foil for 10 minutes while the juices redistribute internally. If the veggies are not tender, rotate them one more time and continue roasting, up to 10 minutes more.
Slice the meat into thin slices against the grain and serve immediately with the veggies.