Place lamb crown roast, rib ends down, on rack in shallow roasting pan. Do not add water. Do not cover. Roast in 375°F oven 20 minutes. Meanwhile, heat butter in large skillet over medium beat until melted. Add onion; cook 4 to 5 minutes or until onion is transparent, stirring occasionally. Add bell pepper, mushrooms and garlic; continue cooking 5 minutes, stirring occasionally. Stir in spinach, salt and pepper. Remove roast from oven. Turn roast so rib ends are up. Insert meat thermometer into thickest part of roast, not touching bone or fat. Fill cavity of roast with spinach stuffing. Return roast to oven; cook 25 to 35 minutes or until meat thermometer registers 140°F for medium-rare to 155°F for medium. Cover roast with aluminum foil tent; let stand 10 to 15 minutes. Roast will continue to rise approximately 5°F in temperature to reach 145°F for medium-rare to 160°F for medium. Trim excess fat from roast; carve roast between ribs.
