Peel the carrots, wash them, and cut them into slices. Wash the leek and celery stalk, then slice them. Peel the onion and stud it with 4 cloves. Peel the garlic cloves and finely dice them. Cut the beef cheeks into large chunks if your butcher has not already done so.
In a large cast-iron Dutch oven or casserole dish, heat a knob of butter and the 3 tablespoons of olive oil. Once hot, sear the beef cheeks thoroughly on all sides until browned, about 10 minutes.
Add all the prepared vegetables, then pour in the beef broth and red wine. Add the bay leaf, thyme, salt, and pepper. Stir well, cover with the lid, and reduce the heat. Simmer gently on low heat for at least 3 hours.
If there is too much liquid toward the end of cooking, remove the lid and simmer uncovered for the final 30 minutes to reduce the sauce.