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.
This was delicious and got rave reviews at my birthday dinner last night. We didn't pound the meat, so it was about 2 inches thick, but my husband carefully grilled it, and there was sections from medium rare to medium available for differing tastes. Yum, I would definitely make this again.