Spaghetti Napolitan (Japanese Ketchup Pasta)

Bring a large pot of water to a boil, then add salt (about 1% of the water's volume, not listed in the ingredients) and stir until dissolved. Cook the spaghetti for 2 minutes longer than the package instructions. At this point, reserve ⅕ cup (per serving) of the pasta cooking water for later use.
Drain the spaghetti in a colander and rinse it under running water to cool and remove excess starch. Once the spaghetti has cooled, drain any excess water, then toss it with oil.
Slice the onion and green bell pepper into pieces about ⅕ inch (5 mm) thick. Cut the sausage diagonally into slices about ⅕ inch (5 mm) thick.
Heat butter in a pan over medium heat. Once the butter has melted, add the onions and sauté until translucent. Then, add the sausages and sauté until slightly browned. Finally, add the green bell peppers and sauté until cooked through. (The reason for adding the ingredients at different times is to ensure that each one is cooked just right.)
Push the ingredients to one side of the pan and add ketchup to the empty space. Cook it for about 1 minute to reduce its acidity.
Stir the ingredients in the pan with the ketchup. Add the spaghetti to the pan, pour the reserved pasta cooking water over it, and cook until the spaghetti is heated through and the water is absorbed while mixing everything together.
Serve on individual plates. Enjoy with grated cheese or Tabasco sprinkled on top, according to personal preference.