Heat 1 tbsp vegetable oil in a large frying pan over medium-high heat.
Add 8 pork sausages and brown on all sides for 5–7 minutes; remove and set aside.
In the same pan, add 2 chopped onions, 2 chopped carrots and 2 chopped celery sticks; sauté for 5–8 minutes until starting to soften.
Stir in 2 crushed garlic cloves and cook for 1 minute until fragrant.
Sprinkle 1 tbsp plain flour over the vegetables and cook for 1 minute, stirring constantly.
Pour in 400g chopped tomatoes and 500ml beef stock; stir in 1 tbsp tomato purée and 1 tsp dried mixed herbs, then season with salt and freshly ground black pepper. Bring to a gentle simmer.
Transfer the sausages and vegetable sauce mixture into the slow cooker and stir to combine.
Cook on LOW for 6–8 hours or on HIGH for 4–5 hours, until sausages are tender and vegetables are soft.
For a thicker sauce, remove the lid for the last 30 minutes of cooking or stir in a cornflour slurry (1 tbsp cornflour mixed with cold water).
Serve hot with creamy mashed potatoes, crusty bread or steamed green vegetables.