Half-fill a large pasta pot with cold water, season very well with salt and add a glug of olive oil. Bring to a rolling boil. In a medium sized saucepan, warm 2 tbsp of extra virgin olive oil over medium/high heat. Add chopped onion, season with a good pinch crushed sea salt flakes and fry for 2-3 minutes until softened then add the garlic and continue to fry for a further 3 minutes, stirring often.
Add the quartered mushrooms and cook, stirring often for 5-7 minutes or until softened and browned. Add the balsamic vinegar and stir into the mushrooms, scraping up any bits in the bottom of the pan. Add the tinned tomatoes and sliced chilli and stir well, season to taste with salt and black pepper. Simmer for 8-10 minutes stirring occasionally.
Whilst sauce is simmering, add spaghetti to the boiling water, cook for 9-11 minutes until al dente. Add 1 cup (250ml) pasta water to the pasta sauce, stir through. Drain spaghetti, add to the pasta sauce, coat well and serve hot with extra freshly ground black pepper.