Preheat oven to 400 degrees.
Pat lamb chops dry and season with salt and pepper. Lightly spray a large, shallow, flameproof casserole or heavy ovenproof skillet (about 12 inches in diameter) with vegetable oil and heat over moderately high heat until hot but not smoking. In casserole, brown lamb chops about 1 1/2 minutes on each side and transfer to a plate. Add onion to casserole and cook, stirring, over moderately low heat until softened. Add tomatoes, garlic, and rosemary and cook over moderate heat, stirring, 1 minute. Stir in broth and beans, and simmer, stirring occasionally, 3 minutes.
Arrange lamb chops on top of bean mixture and roast, uncovered, in middle of oven for 5 minutes for medium-rare. Let lamb chops and bean mixture stand, loosely covered, for 5 minutes. Transfer lamb chops to another plate and toss bean mixture with gremolata.
Serve lamb chops with haricots vertes.