Line a rimmed baking sheet with aluminum foil and place an oven-safe rack inside. Preheat the oven to 425°F (218°C). Trim any excess fat from the boneless leg of lamb.

In a medium mixing bowl, whisk together olive oil, Dijon mustard, minced garlic, dried herbs, salt, and black pepper until emulsified.

Spread one-third of the herb paste inside the opened lamb, roll it up tightly, and secure with kitchen twine. Rub the remaining herb paste all over the outside of the lamb.

Place the lamb in the preheated oven and roast at 425°F (218°C) for 15 minutes.

Reduce the oven temperature to 350°F (177°C) and continue cooking until the internal temperature reaches 135°F for medium-rare, about 1 to 1.5 hours.

Remove the lamb from the oven and let it rest for 15-30 minutes before slicing.