Place your sliced red onions in the base of the slow cooker pot.
Brown the diced beef in a medium hot pan (dry fry, without oil), in two batches. Place it in the slow cooker and add the sliced peppers and crushed garlic.
Fry the chorizo gently in the same pan you used to brown the beef. No oil is needed as the chorizo will produce some as you cook it.
Add the chorizo, passata, and stock cube dissolved in the boiling water and smoked paprika to the slow cooker and combine all the ingredients by mixing with a spoon.
Cook on low for 7-8 hours. Serve garnished with chopped parsley or coriander, and serve with rice, mash or pasta and crusty bread.
Check the stew one hour before the end of cooking, and if the sauce needs thickening, make a paste using a tablespoon of cornflour and a little water, then stir it into the slow cooker pot. Allow it to cook for the remaining time, and it will thicken up.