Combine the ketchup, tonkatsu sauce, and sugar in a small bowl and set aside. Heat a large non-stick pan with 1 tablespoon of olive oil over medium-high heat. Add the garlic and onions. Cook until the onions have softened, about 3 to 5 minutes.

Add the sausage, bell pepper, and mushrooms. Cook until everything is cooked through, about 8 to 10 minutes.

Add the pasta and toss to combine. Pour in the mixed sauce and mix until everything is coated. If the sauce is too thick, add the reserved pasta water to thin it out a bit. Season with salt. Serve with fresh grated parmesan.