Heat 1/4 cup olive oil in a small skillet over medium heat. Add 4 minced garlic cloves and the coriander and cook, stirring often, until the garlic is softened, about 1 minute. Remove from the heat and stir in 1/2 teaspoon red pepper flakes and the lemon zest. Let cool completely.
Generously season the lamb with salt and pepper on both sides, then rub all over with the garlic oil. Let sit at room temperature 1 hour.
Preheat the oven to 275 degrees F. Set a rack in a large roasting pan; set aside. Pulse the olives, 1/4 cup parsley, 1 teaspoon oregano and 1 tablespoon each lemon juice and olive oil in a food processor until a coarse paste forms. Place the lamb fat-side down and spread the olive paste over the top. Starting from a short side, roll the lamb into a log; tie in 1-inch intervals using kitchen twine.
Transfer the lamb, seam-side down, to the rack in the pan. Roast until a thermometer inserted into the center registers 130 degrees F to 135 degrees F, 1 hour 45 minutes to 2 1/2 hours. Let rest in the pan 30 minutes. Increase the oven temperature to 500 degrees F.
Meanwhile, make the salsa verde: Pulse the remaining 2 cups parsley, 1 tablespoon oregano, 1 minced garlic clove, 1/4 teaspoon red pepper flakes and the capers in the food processor until coarsely chopped. Add the remaining 1 tablespoon lemon juice, the vinegar, mustard, 1/4 teaspoon salt and a few grinds of pepper. With the machine running, gradually add the remaining 1/2 cup olive oil until a chunky sauce forms. Season with more salt and pepper.
Return the lamb to the oven and roast until browned, about 20 more minutes. Transfer to a cutting board and let rest 20 minutes.
Remove the twine and cut the lamb into 1/4-inch-thick slices. Transfer to a platter and top with the salsa verde.