Sausages: place a large frying pan on the stove top, turn the heat to high and add the olive oil. Next add the sausages and brown the edges using tongs to rotate sides as they cook. Once browned, turn the heat off and transfer to a paper towel lined plate.
Grease: liberally spray the slow cooker insert with cooking oil.
Sauce: now pour in the beef stock, crushed tomatoes, tomato sauce, bbq sauce, Worcestershire sauce, balsamic vinegar, garlic, mustard powder, paprika, salt and pepper. Then gently stir to combine.
Onions and sausages: next spread the sliced onions across the sauce followed by the browned sausages. Put the lid on the slow cooker and set the timer and temperature to high for 4 hours or low for 8 hours.
Apples: let the dish cook for 2 hours high or 4 hours on low then remove the lid and add the apple slices. Put the lid back on and cook for a further hour (Cook time total 3 hours high or 7 hours low).
Slurry: combine the cornflour and water in a small glass jar and stir until consistently mixed. When there is 1 final hour of cook time to go, remove the slow cooker lid and pour the slurry in. Gently stir to combine and then put the lid back on the slow cooker. Cook for the final hour.
To serve: remove the lid and stir once the cook time is up. Portion onto plates with mashed potatoes, green veggies and garnish with thyme leaves. Seaon with cracked pepper.