Season the bison shanks with salt and pepper. Dredge in flour, tapping off excess.
Heat olive oil in a Dutch oven or heavy pot. Brown shanks on all sides, about 3-5 minutes per side. Set aside.
In the same pot, add onion, carrot, and celery. Cook until softened, about 5 minutes. Add garlic and cook for another minute.
Pour in red wine, scraping up any browned bits. Reduce by half.
Stir in stock, tomatoes, thyme, and bay leaf.
Return shanks to the pot. Bring to a simmer, then cover and cook on low heat for about 2-3 hours, until meat is tender.
Combine lemon zest and parsley in a small bowl to make the gremolata.
Remove the bay leaf, adjust seasoning, and plate the shanks. Spoon over some sauce and vegetables. Garnish with gremolata.
Serve with creamy polenta or mashed potatoes.