Season the flour with salt and pepper to taste. Toss the veal shank slices with the flour until evenly coated. Melt the butter and olive oil in a large skillet over medium heat. Cook the shank slices in the hot butter until golden brown on all sides, about 10 minutes per side.
Combine the onions, carrots, celery, and garlic in a large slow cooker. Set the meat slices atop the vegetables, then pour in the chicken stock and diced tomatoes. Season with oregano and the bay leaf.
Cook on Low for 8 hours until the meat is falling off of the bone. Season to taste with salt and pepper before serving.
