Roughly peel and chop the 2 brown onions/yellow onions, 4 sticks celery, 3 carrots, and 4 cloves garlic, then place them in a food processor. Pulse until finely chopped.
If your slow cooker has a sauté function, set it to high. Otherwise, heat a large frying pan over medium-high heat. Add ½ tablespoon olive oil, then add the 1 kilogram lean beef mince/ground beef, breaking it up with a wooden spoon, until browned.
To the beef, add the finely chopped vegetables and continue to sauté for 5–7 minutes, stirring occasionally, until softened.
Transfer the mixture to the slow cooker if you’re using a frying pan and add in the 2 tablespoons tomato paste, 2 x 400 gram cans crushed tomatoes, 1 beef stock cube, 2 tablespoons dried mixed herbs, 250 ml red wine and 2 bay leaves. Stir well and cover. Cook on high for 4 hours or low for 8 hours.
Once done, remove the lid and cook for another hour on high, or switch the slow cooker back to the sauté function and simmer for 30 minutes until thickened. Stir occasionally to prevent sticking.
Remove the bay leaves, taste, and adjust salt and pepper if needed. Serve over pasta for a classic spaghetti bolognese.