Authentic Spaghetti Napolitan (Japanese Ketchup Pasta)

Bring plenty of water to a boil in a large pot, then add salt (about 1% of the water's volume, not included in the recipe) and mix. Cook spaghetti for 2 minutes longer than the package instructions. At this time, reserve ⅕ cup (per serving) of the pasta cooking water for later use.
Drain the cooked spaghetti in a colander and rinse it under running water to cool and remove excess starch. Once the spaghetti has cooled, drain off any excess water, and then toss it with oil.
Slice onion and green bell pepper into approximately ⅕-inch (5mm) widths. Cut sausage diagonally into slices approximately ⅕-inch (5mm) thick.
Heat butter in a pan over medium heat. Once the butter has melted, add the onions and sauté until they are translucent. Then, add the sausages and sauté until slightly browned. Finally, add the green bell peppers and sauté until cooked through.
Move the ingredients to one side of the pan and add ketchup to the empty space. Heat it for about 1 minute.
Mix the ingredients with ketchup. Add the chilled spaghetti to the pan, sprinkle reserved pasta cooking water over it, and cook until the spaghetti is heated through while mixing everything.
Serve on each plate and sprinkle with grated cheese or Tabasco to taste.