• Gather all the ingredients. Start boiling a large pot of water (I use a 4 QT pot).
  

To Prepare the Ingredients

• In a small bowl, combine ⅓ cup ketchup, ½ Tbsp milk, 2 tsp Worcestershire sauce, 1 tsp sugar. Mix them together and set aside.
  
• Cut ½ onion into thin slices.
  
• Cut ¼–½ green bell pepper in half crosswise (if it‘s long), and then cut into thin strips.
  
• Cut 4 mushrooms into thin slices.
  
• Cut 5 oz sausages diagonally into thin slices.
  

To Boil the Spaghetti

• Once the water is boiling, add 1½ Tbsp Diamond Crystal kosher salt and 7 oz spaghetti. Stir to make sure the noodles don‘t stick to each other. Cook the spaghetti according to the package directions. Toward the end of cooking, reserve some pasta cooking water.

  While the pasta is cooking, prepare the sauce.
  

To Cook the Sauce

• Heat a large frying pan over medium heat. When it‘s hot, add 1 Tbsp extra virgin olive oil and distribute it evenly. Crush and add 1 clove garlic (I use this garlic press) and sauté for 1 minute, just until golden and fragrant.
  
• Add the onions to the pan and sauté for 2–3 minutes.
  
• When the onions are wilted, add the sausages and sauté for 1 minute.
  
• Add the bell peppers and mushrooms and sauté until everything is cooked.
  
• Season with ¼ tsp Diamond Crystal kosher salt and ⅛ tsp freshly ground black pepper.
  
• Push the ingredients to one side and add the sauce mixture to the empty side of the pan. When the sauce starts to bubble and thicken, mix it together with the sausages and vegetables.
  
• Now, scoop out some of the starchy cooking water from the pasta pot, about ¼ cup reserved pasta water. Remember, you may not need the reserved pasta cooking water that you saved. Add it to the pasta sauce only if it needs to loosen up. At times, I do not use it at all or use 1–2 Tbsp. Check the consistency of the sauce, and gradually add reserved pasta water, stirring well, until the sauce is the consistency you like.
  
• Mix it all together and let the sauce simmer and reduce a bit.
  
• By now, the spaghetti should be done cooking. Drain it well in a colander and add it to the pan with the sauce. Add 1 Tbsp unsalted butter and toss to combine using tongs, making sure the sauce coats the pasta evenly.
  

To Serve

• Serve the Napolitan immediately on individual plates. Shave 2 Tbsp Parmigiano-Reggiano or Parmesan cheese over the pasta now or at the table.
  

To Store

• You can keep the leftovers in an airtight container and store them in the refrigerator for 3 days or in the freezer for a month.