Cook the spaghetti or linguine according to the package instructions, but undercook by 1 to 2 minutes. The pasta will continue to cook when mixed into the hot pan with the sauce. Time this to the arrival of your guests for the best result. See notes.
Cut the top off each Japanese green bell pepper and remove the seeds carefully, keeping the pepper shell intact. Cut crosswise into rings — the uneven shape creates charming little florets. If you prefer, simply cut them into strips instead.
In a pan over medium heat, sauté the onion until softened and translucent.
Add the sausages and mushrooms. Cook until nicely browned.
Add the chopped tomato, ketchup, and Worcestershire sauce. Stir to combine and let it cook for a minute.
Add the cooked pasta and Japanese green bell peppers. Season with salt and pepper and toss everything together until well coated.
Plate and garnish with grated Parmigiano-Reggiano or Parmesan cheese and chopped parsley. Serve immediately.