Rub all surfaces of lamb with 1 clove garlic, peeled and halved.
Cut this clove and 1 more into slivers.
Slit skin of lamb at intervals over the top surface; insert slivered garlic into slits.
Blend dry mustard, salt, pepper, thyme, rosemary, and lemon juice.
Spread over surface of the roast.
Coat potatoes well with butter; place in pan with roast.
Place lamb on rack in an uncovered roasting pan in a 325° oven and roast for 2 to 2 1/2 hours or longer, until thermometer in thickest part registers 150° (for medium rare); 160° (medium well done); or 175° (well done).
Baste occasionally with pan drippings.
Remove meat and potatoes to platter; keep warm.
For sauce, skim fat from pan drippings; reserve 1 tablespoon.
Stir flour into fat in pan; heat until bubbly.
Add water to skimmed drippings to make 1 1/2 cups; blend into flour mixture, stirring constantly until thickened.
Simmer for 3 to 5 minutes; strain to remove crusty particles.