In a small bowl, combine oil, coriander and fennel seeds, red-pepper flakes, 1 teaspoon salt, and 1/4 teaspoon pepper. In a shallow dish, coat lamb with marinade. Marinate at room temperature for 30 minutes (or refrigerate, up to overnight).
Heat grill to medium-low; lightly oil grates. Grill lamb, covered, until an instant-read thermometer inserted in thickest part of meat registers 130 degrees for medium-rare, 6 to 8 minutes per side. Remove from grill. Tent with aluminum foil; let rest 5 minutes. Thinly slice lamb, and serve with lemon wedges, if desired.
While the lamb is marinating, cook up a simple, refreshing Mediterranean-style side, such as our lemony orzo, to balance the richness of the meat.