In a heavy skillet, heat the olive oil and butter over medium heat. Brown the lamb, a few pieces at a time. Transfer with a slotted spoon to a deep ovenproof casserole.
Bring 2 quarts of salted water to a boil. Cut a shallow 5 in the root end of each pearl onion and drop them into the water. Cook until tender but firm, 10 minutes. Drain, transfer to a small bow