Heat your grill to high and line a baking tray or grill pan with foil. Halve the peppers and scoop out the seeds. Lay them on the tray, cut-side down, and grill for 15 mins till they’re charred and black. Pop in a bowl, cover with cling film or a plate and let rest for 10 mins.
While the peppers grill, peel and very finely slice the onions. Halve the cherry and baby plum tomatoes. Peel and finely chop the garlic. Halve the chilli, scoop out the seeds and white pith, and finely chop the chilli.
Put the onions, tomatoes, garlic and chilli in a large pan. Add the spaghetti to the pan.
Peel as much charred skin as you can off the peppers. Roughly chop the peppers and add them to the pan. Crumble in the vegetable stock cube and pour in the cherry tomato passata. Add 1 tsp sea salt, a good grinding of black pepper, 2 tbsp olive oil and 1 ltr boiling water.
Pop a lid on the pan and bring to the boil, then turn the heat down and simmer for 20 mins. Use 2 forks or some tongs to stir, pick up and mix the spaghetti a few times while it’s cooking to stop everything sticking together. The spaghetti should end up tender but not too still soft – it should still have some bite to it.
Take the dish off the heat. Toss the spaghetti and sauce to make sure they’re well mixed. Taste the one-pot and adjust the seasoning if needed. Heap it into warm bowls to serve.