Preheat oven to 375 degrees. Using a chef's knife, finely chop garlic and bay leaves together. Gather into a pile; sprinkle with 2 teaspoons coarse salt and 1/2 teaspoon ground pepper. Using the flat side of the knife blade, mash mixture into a paste. Set aside.
On a large rimmed baking sheet, toss onions with 2 tablespoons oil; season with salt and pepper. Push onions to the edges of baking sheet.
Place pork in center of sheet, fatty side up; rub top with remaining tablespoon oil, and press on garlic mixture, coating evenly.
Roast, dabbing occasionally with pan juices, until an instant-read thermometer inserted in center of meat (avoiding bones) registers 140 degrees (temperature will rise 10 to 15 degrees as roast rests), 65 to 75 minutes. (If browning too quickly, tent loosely with aluminum foil.) Transfer roast and onions to serving platter; let rest, loosely covered with foil, about 10 minutes.
Meanwhile, in a food processor, blend parsley and mustard until smooth; season with salt and pepper.
Cut roast into chops, and serve with mustard sauce.
Ask the butcher to leave some fat on top of the pork to keep it moist during roasting, and, for easier carving afterward, to remove the chine bone (or backbone). For an elegant presentation, have him "French" the rib bones, which means to scrape off the gristle and fat.