Napolitan (Japanese Ketchup Spaghetti)

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• 5 oz sausages (4–6 pieces; see Notes for substitute)
  Ingredient note

  Source: 5 oz sausages (4–6 pieces; see Notes for substitute)
• 4 each mushrooms (2.5 oz, 70 g)
  Ingredient note

  Source: 4 mushrooms (2.5 oz, 70 g)
• 0.5 each green bell pepper (2.5 oz, 70 g)
  Ingredient note

  Source: ¼–½ green bell pepper (2.5 oz, 70 g); upper source range endpoint used for shopping.
• ½ each yellow onion (3.5 oz, 100 g)
  Ingredient note

  Source: ½ yellow onion (3.5 oz, 100 g)
• 1 clove garlic
  Ingredient note

  Source: 1 clove garlic
• ¼ tsp + 1½ Tbsp Diamond Crystal kosher salt (use half the amount or the same weight for table salt)
  Ingredient note

  Source: ¼ tsp Diamond Crystal kosher salt (use half the amount or the same weight for table salt); Source: 1½ Tbsp Diamond Crystal kosher salt (use half the amount or the same weight for table salt)
• ⅛ tsp freshly ground black pepper
  Ingredient note

  Source: ⅛ tsp freshly ground black pepper
• 1 Tbsp extra virgin olive oil
  Ingredient note

  Source: 1 Tbsp extra virgin olive oil
• ⅓ cup ketchup
  Ingredient note

  Source: ⅓ cup ketchup
• 2 tsp Worcestershire sauce
  Ingredient note

  Source: 2 tsp Worcestershire sauce
• ½ Tbsp milk (optional)
  Ingredient note

  Source: ½ Tbsp milk (optional)
• 1 tsp sugar (optional)
  Ingredient note

  Source: 1 tsp sugar (optional)
• 7 oz spaghetti
  Ingredient note

  Source: 7 oz spaghetti
• 1 Tbsp unsalted butter
  Ingredient note

  Source: 1 Tbsp unsalted butter
• ¼ cup pasta water (reserved; use as needed)
  Ingredient note

  Source: ¼ cup pasta water (reserved; use as needed)
• 2 Tbsp Parmigiano-Reggiano or Parmesan cheese
  Ingredient note

  Source: 2 Tbsp Parmigiano-Reggiano or Parmesan cheese

1. Gather all the ingredients. Start boiling a large pot of water (I use a 4-QT heavy-bottomed pot).
2. Add ⅓ cup ketchup, 2 tsp Worcestershire sauce, ½ Tbsp milk (optional), and 1 tsp sugar (optional) to a small bowl. Mix together and set aside.Nami's Tip: Both are optional, but the milk makes the sauce creamier and the sugar gives Napolitan its characteristic sweetness.
3. Thinly slice ½ yellow onion and 4 mushrooms. Cut 5 oz sausages diagonally into thin slices.
4. Cut ¼–½ green bell pepper in half crosswise (if it's long), and cut into thin strips.
5. Add 1½ Tbsp Diamond Crystal kosher salt and 7 oz spaghetti to the boiling water. Stir to keep the noodles from sticking to each other. Cook according to the package directions.Make the sauce while it cooks so both finish together. Before draining, reserve ¼ cup pasta water. Drain the spaghetti when cooked and add it to the sauce promptly. Nami's Tip: Napolitan isn't served al dente. Cook the spaghetti for the full package time or a minute past it—the softer texture is part of the dish.
6. Heat a large frying pan over medium heat. Add 1 Tbsp extra virgin olive oil and distribute it evenly. Crush 1 clove garlic with a garlic press and add it to the pan. Sauté for 30 seconds until fragrant.Add the onions and sauté for 2–3 minutes until tender.
7. Add the sausages and sauté for 1 minute, then add the bell peppers and mushrooms.
8. Cook, stirring, until the sausages are heated through and the vegetables are tender. Season with ¼ tsp Diamond Crystal kosher salt and ⅛ tsp freshly ground black pepper.
9. Push the ingredients to one side and add the sauce mixture to the empty side of the pan. Let it bubble and thicken for about 30 seconds, then mix with the sausages and vegetables.Nami’s Tip: Cooking the sauce before mixing it with the other ingredients mellows the ketchup’s tanginess and brings out its flavor for that classic kissaten taste.
10. Scoop ¼ cup pasta water from the pot. If the sauce looks too thick, stir in 1–2 Tbsp of pasta water to loosen it. Add more if needed.Nami's Tip: The starchy cooking water helps the sauce come together and coat the spaghetti. Add a splash as needed to loosen it.
11. Mix to combine, and let the sauce simmer and reduce a bit. Once the spaghetti is done, drain it and add it to the pan with the sauce. Add 1 Tbsp unsalted butter and toss with tongs until the pasta is evenly coated.Nami's Tip: If the sauce tastes too tangy, add a little more sugar to taste. The butter added next brings richness and rounds out the flavor.
12. Serve immediately on individual plates. Grate 2 Tbsp Parmigiano-Reggiano or Parmesan cheese over the pasta now or at the table.
13. Napolitan is at its best straight out of the pan, so it's worth cooking only what you'll eat. If you do have leftovers, store them in an airtight container in the refrigerator for up to 3 days, or in the freezer for up to a month.