Make small incisions all over the lamb with a sharp knife. Insert slices of garlic into the incisions.
Rub the lamb with olive oil, rosemary, thyme, salt, and pepper.
Place the lamb on a rack in a roasting pan.
Prepare the Vegetables:
In a large bowl, toss the potatoes, carrots, and onion with olive oil, salt, pepper, and dried thyme.
Spread the vegetables around the lamb in the roasting pan.
Roast:
Roast the lamb and vegetables in the preheated oven for about 1.5 to 2 hours, or until the internal temperature of the lamb reaches your desired doneness:
135°F (57°C) for medium-rare
145°F (63°C) for medium
Stir the vegetables halfway through the cooking time to ensure they cook evenly.
Rest the Lamb:
Remove the lamb from the oven and let it rest for 15-20 minutes before carving. This allows the juices to redistribute, making the meat more tender and flavorful.
Make the Gravy (optional):
While the lamb is resting, place the roasting pan with the vegetables on the stovetop over medium heat.
Sprinkle the flour over the vegetables and pan juices, stirring to combine.
Gradually add the broth and red wine, stirring constantly until the gravy thickens, about 5-7 minutes.
Strain the gravy into a serving bowl, if desired.
Serve:
Carve the lamb and serve it alongside the roasted potatoes, carrots, and onion.