Special equipment: an 8-quart stovetop pressure cooker
Cut the leg of lamb into 4 even pieces.
Heat 2 to 3 tablespoons olive oil in an 8-quart stovetop pressure cooker over medium-high heat. Sprinkle the lamb pieces and bones all over with salt and pepper. Working in batches (to not overcrowd the pot), sear the meat and bones in the hot oil until well browned on all sides. Remove from the pot and set aside. Pour off excess oil from the pot and then add the garlic, fennel, jalapenos, onions and red bell peppers. Sweat for 2 to 3 minutes. Add tomato paste; cook for 2 minutes. Deglaze with the beef stock and balsamic vinegar. Add the seared lamb and bones back the pot, close the pressure cooker and cook on low heat until fork tender, 45 minutes.
Remove the lamb from the braising liquid and pull into large chunks. Remove the bones from the pot, and discard. Strain the braising liquid and then add it back to the pot. Cook over medium heat until the liquid is reduced by half. Turn off the heat, add the lamb back to the pot and spoon the reduced liquid over the top to glaze. Keep warm until ready to serve.