Spaghetti Napolitan with Ketchup-Shoyu Butter and Sausage

Why You’ll Love This Recipe

• Weeknight-fast, with big nostalgic flavor from a ketchup–shoyu–butter sauce.
• Balanced textures: springy spaghetti, tender onions, snappy peppers, and browned sausages.
• Picky-eater friendly yet full of umami; pantry staples do the heavy lifting.
• One-pan finish for a restaurant-style glossy coating.

Grocery List

• Produce: 1 large onion, 1 large green bell pepper, 8 oz mushrooms
• Dairy: Unsalted butter, Parmesan cheese
• Pantry: Spaghetti, ketchup, soy sauce, Worcestershire (optional), neutral oil, kosher salt, black pepper

Full Ingredients

Pasta

• 14 oz (400 g) spaghetti
• 4 qt (3.8 L) water + 2 tbsp kosher salt

Ketchup–Shoyu Butter Sauce

• 2/3 cup (160 g) ketchup
• 1 tbsp (15 ml) soy sauce (shoyu)
• 1 tsp Worcestershire sauce (optional, for depth)
• 2 tbsp (28 g) unsalted butter, cold and cubed (to finish)
• Up to 1 cup reserved pasta water (you may not need it all)

Sauté and Finish

• 1 tbsp neutral oil (canola or grapeseed)
• 1 tbsp (14 g) unsalted butter
• 1 large yellow onion (about 10 oz/280 g), thinly sliced
• 1 large green bell pepper (about 7 oz/200 g), seeded and thinly sliced
• 8 oz (225 g) cremini or button mushrooms, sliced
• 6 oz (170 g) Japanese-style sausages or hot dogs, sliced 1/4-inch on the bias
• Freshly ground black pepper, to taste (start with 1/2 tsp)
• 1/2 cup (30 g) finely grated Parmesan cheese, plus more for serving

Step-by-Step Instructions

Step 1: Boil the pasta water and prep the produce

Bring 4 quarts (3.8 L) of water to a boil and season with 2 tablespoons kosher salt. While it heats, slice the onion, bell pepper, mushrooms, and sausages. Measure the ketchup and soy sauce. Cut 2 tablespoons butter into small cubes and chill them for finishing later.

Step 2: Mix the ketchup–shoyu base

In a small bowl, whisk together the ketchup and soy sauce (and Worcestershire if using). This concentrated base will be stretched with starchy pasta water to coat the noodles evenly.

Step 3: Cook the spaghetti

Add spaghetti to the boiling water and cook until 1 minute shy of al dente (8–10 minutes, or per package). Before draining, reserve 1 cup of the pasta water. Drain the pasta and keep it handy; it will finish in the pan with the sauce.

Step 4: Brown the sausages

Heat a large 12-inch skillet over medium-high heat. Add 1 tablespoon neutral oil and 1 tablespoon butter. When sizzling, add the sliced sausages and cook, stirring, until the edges are browned and lightly blistered, 2–3 minutes.

Step 5: Sauté the vegetables

Add the onions and mushrooms to the skillet. Sauté, stirring occasionally, until the onions are translucent and mushrooms are lightly golden, 4–5 minutes. Add the bell pepper and cook 2 minutes more; it should stay crisp-tender for the classic Napolitan bite.

Step 6: Build the sauce and toss the pasta

Pour the ketchup–shoyu mixture into the skillet along with 1/2 cup of the reserved pasta water. Bring to a brisk simmer for 30–60 seconds, scraping up any browned bits. Add the drained spaghetti and toss vigorously with tongs over medium heat until the sauce clings and the pasta finishes cooking, 1–2 minutes. Add more pasta water a splash at a time if it looks dry.

Step 7: Finish with butter, pepper, and Parmesan

Take the pan off the heat and immediately swirl in the 2 tablespoons of cold butter until the sauce is glossy. Grind in black pepper to taste. Plate the pasta, then generously sprinkle with finely grated Parmesan. Serve right away, with extra cheese and pepper at the table.

Pro Tips

• Undercook the pasta by about 1 minute; it will finish in the skillet and absorb the sauce better.
• Keep 2 tablespoons of butter cold so it emulsifies and glosses the sauce at the end.
• Do not overcook the bell pepper—crisp-tender strips are key to Napolitan’s texture.
• Use high heat and avoid crowding the pan so mushrooms brown instead of steam.
• Reserve pasta water; its starch helps the ketchup–shoyu base cling to the noodles.

Variations

• Vegetarian: Skip the sausages and add more mushrooms; season with 1/4 teaspoon smoked paprika for depth.
• Creamy Napolitan: Stir in 2–3 tablespoons heavy cream with the pasta water for a softer, richer sauce.
• Spicy: Add 1/2–1 teaspoon shichimi togarashi or a pinch of red pepper flakes when sautéing the onions.

Storage & Make-Ahead

Leftovers keep in an airtight container in the refrigerator for up to 3 days. Reheat gently in a skillet over medium heat with a splash of water to loosen the sauce; add a small knob of butter if desired. The ketchup–shoyu base can be mixed up to 1 week ahead and refrigerated. Freezing is not recommended, as the vegetables can soften and the sauce may separate.

Nutrition (per serving)

Approximate: 680 calories; 28 g fat; 90 g carbohydrates; 22 g protein; 4 g fiber; 1400 mg sodium. Values will vary based on sausage, ketchup, and cheese brands.