For the crumble: Roll the mint leaves and thinly slice, then in a bowl blend the mint, feta, red onion and lemon zest. Then season with salt and pepper.
For the osso bucco: Preheat the oven to 350 degrees F. In a heavy-bottomed pan heat the vegetable oil to medium-high heat, then sprinkle the lamb with salt and pepper. Next sear the meat on each side until brown, 2 to 3 minutes per side.
Add the stock, carrots, celery, onions and tomato paste to the pan. Tightly cover the pan with foil, and cook in the oven for 4 hours.
Remove the pan from the oven, then check the tenderness of the meat, it should be fork tender. If need be, continue cooking to achieve desired tenderness. Then remove the ribs and save the remaining liquid. Over medium-high heat reduce the braising liquid until the liquid has reduced to 2 cups.
Serve the osso bucco with mashed potatoes, then drizzle with sauce and finish with feta crumble.