Put pasta on to boil.
Remove skin from sausage. Fry with a little oil in a large frypan, and break up well with masher.
Add diced tomato to pan, and when softened in a few minutes, break this up with masher too, ensuring it's well mixed with the sausage (see photos above).
Cook pasta under just under cook time on packet directions. Drain, reserving about half a cup of the salted pasta water.
Add drained pasta water and pasta to the frypan. Stir continuously so the sauce seeps into the pasta by osmosis for a a minute or so.
When no sauce left to seep, stir through parsley, parmesan, salt and pepper.