Bring a large pot of water to a boil and cook the pasta for one minute less than package instructions.
Drain the pasta but reserve approximately 2 cups (500 ml) of pasta cooking water.
While pasta cooks, heat oil in a large frying pan over medium heat.
Add diced onion and sauté for about 5 minutes until softened.
Add minced garlic, salt, and black pepper; cook and stir for 30 seconds.
Stir in gochujang paste, tomato puree, and soy sauce; cook for 2 minutes, stirring frequently.
Add grated Parmesan and heavy cream; stir until combined and creamy.
Add half of the reserved pasta water to the sauce and stir well.
Add drained pasta and butter to the pan; toss to coat pasta evenly in the sauce.
Cook together for 1 minute, adding more pasta water if needed to loosen sauce.
Divide pasta between four bowls and garnish with spring onions, extra Parmesan, and sesame seeds.