Lamb Pot Roast with Oranges and Olives

Holding knife parallel to work surface, cut lamb lengthwise, following seam where bone was removed and stopping about 1/2" before you get all the way through. Open lamb like a book and season all over with salt and pepper. Close, wrap tightly in plastic, and chill at least 8 hours. Preheat oven to 400 °F. Open lamb and arrange garlic on bottom half, leaving a 1" border. Top with orange slices and olives. Sprinkle with savory and red pepper flakes. Close lamb and tie at 1" intervals with kitchen twine. Place lamb on a wire rack set inside a large rimmed baking sheet. Roast until nicely browned, 30 –40 minutes. Remove lamb from oven. Reduce oven temperature to 300 °F. Carefully transfer lamb to a large heavy ovenproof pot and add stock, wine, and tomato purée. Cover and braise lamb, basting with braising liquid every 30 minutes, until golden brown and fork-tender, 2 1/2 –3 hours. Season with salt and pepper, if needed. Transfer lamb to a cutting board and let rest at least 30 minutes before slicing. Serve with braising liquid. DO AHEAD: Lamb can be seasoned 1 day ahead; keep chilled. Lamb can be braised 2 days ahead; cover and chill.