Bring a large pot of salted water to a boil. Cook the spaghetti — for 2.2mm pasta, around 16 minutes. Cook through but not mushy.
Heat oil in a wide frying pan over medium-high heat. Add sausage and brown lightly. Then add the onion, carrot, shimeji, and green pepper. Stir-fry for 4–5 minutes until the onion is translucent. Season with salt and pepper.
Push the vegetables to the sides and add the ketchup directly into the middle. Let it sit for 10–15 seconds to caramelise. Stir in the Worcestershire sauce.
Drain the pasta and add directly to the pan. Toss to coat. If dry, add a splash of pasta cooking water.
Drizzle a little extra ketchup over the top and fold it in. Add the butter and toss until melted into the sauce.
Plate immediately and top generously with parmesan cheese. Eat while hot.
