In a large pan over medium-high heat add a drizzle of olive oil then place in your sausages. Fry until deep golden all over, turning frequently, then remove from the pan leaving the fat behind. Rest then slice just before needed. Don't worry about cooking the sausages right through, this is just about browning the outside and rendering down the fat.
Add in peppers & onion and fry until they begin to soften and lightly brown. Lower heat slightly, then make space in the centre for the garlic and fry for 1-2mins until it just begins to pick up colour. Stir in tomato paste and fry for another couple of minutes.
Pour in tomatoes then fill one of the cans about half the way up with water. Use this to swill out the cans and pour that in too. Add in oregano, basil, salt, pepper and chilli flakes. Give it a stir and burst some of the tomatoes with your wooden spoon. Stir in the slices of sausage then lower heat to a gentle simmer. Cook for 30mins, stirring occasionally & gently bursting the chunks of tomato as needed. This time is important to reduce the sauce and marry all the flavours together.
Meanwhile, pop your pasta in heavily salted boiling water and cook until al dente. Retain a cup of the pasta water just before draining.
Stir pasta into the sauce, then sprinkle in parmesan & parsley and give it all another good stir. If you need to thin out the sauce slightly just add a splash of starchy pasta water (discard otherwise).
Serve up with extra parmesan & parsley!