Vermicular | Spaghetti Napolitana

Ingredients

• 6 oz (160 g) dried spaghetti
• 2 qt water
• 1 tablespoon kosher salt
• ½ medium onion, cut into wedges
• 6 sausages, cut into ½” diagonal slices
• ½ cluster shimeji mushrooms, stemmed and separated
• 2 green bell peppers, stemmed and seeded, cut into ⅓” slices
• ½ teaspoon kosher salt
• Freshly ground black pepper, to taste
• 4 tablespoons ketchup
• 1 tablespoon vegetable oil