Spray a large frying pan with low-calorie cooking spray and set on a medium heat. Add the meatballs and fry for 8-10 minutes until browned on all sides. Remove to a plate.
Re-spray the pan and add the onions, then fry for 5 minutes. Add the mushrooms and continue to fry for a further 5 minutes. The onion and mushrooms should be softened and golden brown.
Add the garlic, rosemary, thyme, balsamic, Henderson’s relish, and tomato puree. Stir well.
Add the stock cube to the water and stir until dissolved. Pour into the pan. Bring the pan to the boil, then lower the heat.
Mix the cornflour with 1 tbsp of water until smooth. Add to the pan and stir well.
Add the meatballs back to the pan. Cover with a lid and cook for 30 minutes. The meatballs should be cooked through, and the sauce thickened and reduced. Season to taste and serve.