Pre-heat the oven to 200°c.
Put the onion in the food processor and pulse until finely chopped.
Spray a frying pan with low calorie cooking spray, add the onion and the stock pot (without water) and gently fry for 5 minutes until the onion has softened.
While the onion is cooking, pulse the 100g of mushrooms for the meatballs in the food processor until finely chopped. Transfer them into a separate bowl with the oats.
Add the beans, marmite and tomato puree to the food processor and pulse until you have a chunky paste.
Pour the bean paste into the bowl with the mushrooms and add the cooked onion, paprika, onion granules and garlic granules. Mix to combine and season with salt and pepper to taste. The mix will be sticky and thick, but not runny.
Line a baking tray with foil and spray with low calorie cooking spray.
Shape the mixture into meatballs and place on the baking tray. You are aiming for around 50-60 small meatballs that are about 2cm in diameter each (This is about a heaped teaspoon of mix). The mixture will be sticky so be gentle when forming the meatballs.
Once formed, spray the tops of the meatballs with low calorie cooking spray and place in the middle of the oven for about 30 minutes. When cooked, the outsides will be firm but the insides will be a little springy. When you remove the meatballs from the oven let them cool for 5 minutes before gently removing from the baking tray. This will help prevent them sticking and breaking apart.
While the meatballs are cooking, prepare the pasta and sauce. In the frying pan add the onion and red pepper and sauté gently over a medium heat for 10 minutes until softened.
Add the 150g of mushrooms for the sauce and balsamic to the pan and fry for 2 minutes until the mushrooms have softened and coloured.
Add the chopped tomatoes, garlic granules, basil and oregano. Stir in the tomato puree and turn down to simmer for 15 minutes while the meatballs finish cooking. Season with salt and pepper to taste.
Add the dried spaghetti to a pan of boiling water and cook for 10 minutes. Drain and serve with the sauce, meatballs and optional fresh basil torn and scattered over the top.