In the bowl of a food processor, make a marinade: Combine garlic, half the parsley, plus onion, mustard, rosemary, lemon peel, and 4 tablespoons olive oil, until finely chopped and emulsified.
In a medium-size bowl, combine lamb and marinade. Cover and refrigerate at least 4 hours or overnight. Then heat your oven to 350 degrees. In a heavy-bottomed casserole with high sides, heat remaining olive oil. Add anchovies and mash with a fork; then add red-pepper flakes (if you like). Add lamb (discarding marinade), and brown on all sides. Chop remaining parsley. Add white wine, vinegar, and 1 tablespoon parsley; bring to a boil. Turn down the heat and finish cooking 1 hour in the oven. When lamb is tender, remove to a plate, reserving sauce. Keep warm.
Place casserole on stovetop over medium-high heat. Sprinkle flour over sauce, and stir well. Add remaining parsley. When sauce thickens slightly, return lamb to casserole and stir. Serve immediately with couscous, fregula, or rice.