Braising lamb shanks makes them meltingly tender. Serve with Polenta for a hearty meal.
Preheat oven to 375 degrees. Season lamb all over with salt and pepper. Place flour in a shallow bowl. Dredge lamb in flour, turning to coat evenly; shake off excess. Heat oil and butter in an 8-quart Dutch oven or skillet over medium heat. Working in two batches, cook until shanks are well browned, about 5 minutes per side. Transfer shanks to a large plate; drain off all but 2 tablespoons fat from the skillet.
Place skillet over medium heat; add onions and leek, and saute until they are lightly browned, about 6 minutes. Add garlic; cook 4 minutes more. Add red wine; deglaze pan by scraping up any browned bits with a wooden spoon. Return shanks to skillet, and add chicken stock, tomatoes, tomato paste, thyme, and bay leaves. Bring to a boil; cover, and place in oven. Braise 2 hours; add fennel, and cook 30 minutes.
Using a slotted spatula, transfer shanks and vegetables to a large bowl. Cover; set aside. Using a ladle, skim fat from surface of cooking liquid; cook over medium heat until liquid is thickened, 5 to 7 minutes. Return shanks and vegetables to pan; cook until heated through. Remove from heat, and season with salt and pepper.