Remove the fell (tissue-like covering) from lamb with a sharp knife. Trim excess fat. Rub opening formed from boning with onion, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Combine breadcrumbs, butter, chilies, 1 teaspoon salt, thyme, and paprika. Spoon into opening. Fold lamb over stuffing; fasten with skewers, and tie securely with string.
Dredge lamb in flour; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Place in a shallow roasting pan. Pour water around lamb. Insert meat thermometer, if desired.
Bake, uncovered, at 400° for 30 minutes; reduce heat to 350°. Bake until desired degree of doneness: about 1 hour or 140° (rare); about 1 1/2 hours or 160° (medium); about 2 hours or 170° (well done). Transfer lamb to a warm serving platter. Let stand 10 minutes before slicing.