01 - Preheat your oven to 400°F (200°C) with rack positioned in the center.
02 - Using a sharp knife, make small, deep incisions all over the surface of the leg of lamb. Insert the sliced garlic cloves into each slit, pushing them in completely.
03 - In a small mixing bowl, combine the chopped rosemary, thyme, Dijon mustard, olive oil, sea salt, black pepper, and lemon zest. Stir until well blended into a paste-like consistency.
04 - Rub the herb mixture thoroughly over the entire leg of lamb, pressing it into the meat to ensure even coverage. Allow to sit at room temperature while preparing the vegetables.
05 - Arrange the onion slices in a single layer at the bottom of a large roasting pan. Place the baby potatoes around the onions, then drizzle with 1 tablespoon olive oil and toss to coat evenly. Season with additional salt and pepper.
06 - Place the seasoned leg of lamb directly on top of the onions. Pour the white wine or chicken broth into the bottom of the pan, being careful not to wash the seasoning off the lamb.
07 - Roast for 1 hour 30 minutes. Baste the lamb with pan juices once or twice during cooking. After 45 minutes, gently toss the potatoes with a spoon for even browning.
08 - Insert a meat thermometer into the thickest part of the lamb (without touching bone). The internal temperature should reach 130–135°F for medium-rare. Remove from oven when target temperature is reached.
09 - Transfer the lamb to a cutting board and tent loosely with aluminum foil. Let rest for 15 minutes to allow juices to redistribute. Keep potatoes warm in the turned-off oven if desired.
10 - Carve the lamb into thick slices against the grain. Serve with roasted potatoes and onions, spooning pan juices over the top.