Heat the 1 tablespoon olive oil in a large frying pan over medium-high heat. Brown the 800 grams thick beef sausages on all sides, then transfer them to the slow cooker.
In the same pan, add the 1 large brown onion/yellow onion and cook for 2–3 minutes, until softened and lightly golden. Transfer to the slow cooker with the sausages.
In a bowl or jug, mix together the 2 garlic cloves, grated 1 Granny Smith apple, 3 tablespoons tomato paste, 2 tablespoons Worcestershire sauce, 1 tablespoon Dijon mustard, 1 tablespoon apple cider vinegar, 2 tablespoons brown sugar, 500 ml beef stock, ½ teaspoon onion powder, ½ teaspoon garlic powder and ¼ teaspoon black pepper until combined.
Pour the sauce over the sausages and onion in the slow cooker and stir gently to combine.
Cover and cook on LOW for 5–6 hours or HIGH for 3–4 hours, until the sausages are tender and the sauce is rich.
Mix the 2 tablespoons cornflour/cornstarch with the cold water to make a slurry and stir it through the sauce. Turn the slow cooker to HIGH and cook with the lid off for 10–15 minutes, until the sauce has thickened.
Serve the sausages over Dairy free mashed potato with Steamed peas.