Preheat oven to 350F and arrange a roasting rack in a roasting pan.
Unroll leg of lamb on a cutting board so it is completely flat and use a meat mallet to pound it out to 1 inch thickness. Season all over on both sides with salt and pepper. Place lamb skin-side down on cutting board and spread Dijon mustard, garlic, and rosemary in an even layer over lamb.
Starting with the short side, tightly roll up lamb and use kitchen twine wound around lamb to secure in place.
In a large skillet or Dutch oven, heat butter and olive oil over medium-high heat. Once pan is hot, add lamb and sear on all sides, gently turning occasionally until lamb is golden-brown. Transfer lamb to roasting rack and pour water in the bottom of roasting pan (be careful to not splash the lamb)
Roast lamb at 350F 60-75 minutes (about 20 minutes per pound of meat) until a kitchen thermometer inserted in the thickest part of lamb registers 130F.
Remove lamb from oven, transfer to a cutting board, lightly tent with foil, and allow to rest 20 minutes.
Once lamb has rested, use a knife to remove twine and cut lamb into slices for serving. Serve lamb warm and enjoy!