Remove and discard seeds from preserved lemons. Coarsely chop 1 preserved lemon; transfer to a food processor. Add rosemary, mint, garlic, 2 teaspoons salt, and 1/2 teaspoon pepper. Process until coarsely ground. With processor running, pour in oil in a slow, steady stream; process until a thick paste forms.
Spread herb mixture all over lamb; transfer to a 9-by-13-inch baking dish. Cover; refrigerate 8 hours (or overnight).
Preheat oven to 450 degrees. Season lamb with salt and pepper. Cut remaining preserved lemon into thin strips; arrange strips over one side of lamb. Starting from one narrow end, roll up lamb. Using kitchen twine, tie lamb crosswise at 1-inch intervals. Season with salt and pepper.
Transfer lamb to a roasting pan fitted with a rack just large enough to hold it. Roast in middle of oven for 25 minutes. Add 1/2 cup water to pan; reduce heat to 400 degrees. Continue roasting until an instant-read thermometer inserted into thickest part of meat registers 125 degrees to 130 degrees (for medium-rare), about 35 minutes.
Tent with foil. Let rest 25 minutes (internal temperature will rise). Transfer to a cutting board; remove twine. Cut into thin slices. Serve with lemon wedges.