In a large skillet, heat the oil over medium-high heat. Add the beef cubes, in batches without crowding, and cook, turning occasionally, until browned on all sides, adding more oil if necessary. Transfer the beef cubes to a 3-1/2-quart slow cooker and season with the salt and pepper.

Add more oil to the skillet, if necessary, and heat over medium heat. Add the onions and cook until softened, about 4 minutes. Add the garlic and stir for 1 minute. Stir in the red wine, tomato paste, pickling spices, and bay leaf. (You may prefer to use the pickling spices in a tea ball, or tie them up in a piece of rinsed cheesecloth, but I usually skip this refinement.) Bring to a simmer, scraping up the browned bits on the bottom of the skillet. Remove from the heat and stir in the tomatoes with their juices, breaking them up with the side of a spoon. Transfer the mixture to the slow cooker.

Cover and slow-cook until the meat is tender, 6 to 7 hours on low (200°F). Skim the fat from the surface of the stew and discard the bay leaf. If using dried mint, stir into the stew.

Place the bread chunks in a large serving bowl. Pour the stew over and let stand for 5 minutes. Sprinkle the stew with the fresh mint, if using, and serve immediately.