Rub both sides of lamb shoulder chops with 1 to 2 garlic cloves, as needed. Rub with chopped herbs and season with salt and pepper. Dust both sides of chops with flour. In a large (10-inch) cast iron skillet, heat oil over high heat. When nearly smoking, add 2 lamb chops and sear, 4 minutes each side, until well-browned. Remove and repeat with remaining chops. Discard fat that has accumulated in pan.
In skillet layer onions, half of tomato slices and lamb chops. Top with remaining tomato slices and pour wine and 1/2 cup water over. Bring to a boil over medium-high heat. Add remaining 4 garlic cloves, bay leaves, thyme sprigs and parsley stems. Cover pan loosely with aluminum foil and roast for 3 hours.
Cover top with potato slices, pushing them into liquid so they are completely submerged. Cover pan, return to oven and cook 30 minutes or until potatoes are tender. Using a large spoon, remove excess liquid and any fat from stew to a small saucepan. Bring liquid to a boil over high heat and boil rapidly until thickened, 10 to 15 minutes. Before serving, broil lamb under a preheated broiler, 5 minutes or until potatoes are well-browned. Sprinkle with chopped parsley and serve with sauce.