Combine oil, mint, garlic, zest, 3 tablespoons salt, and the red-pepper flakes; season with pepper. Reserve 1/2 cup marinade.
Pound lamb to an even 1-inch thickness. Place lamb in a baking dish, and massage marinade into meat. Refrigerate overnight.
Let lamb stand at room temperature for 1 hour before grilling. Heat grill to medium-high. Brush grates with oil. Remove lamb from dish, and brush off any excess marinade. Grill lamb, flipping every 5 minutes for even cooking, until an instant-read thermometer inserted into the center registers 130 degrees to 135 degrees for medium-rare, 8 to 10 minutes per side. Let stand for 10 minutes. Slice lamb across the grain. Drizzle with reserved marinade.