Boil a large pot of salted water and cook the thin spaghetti according to package instructions until al dente. Reserve ¾ cup of hot pasta water, drain the spaghetti, and set aside.

Heat 2 tablespoons of olive oil and 1 tablespoon of butter over medium-low heat in a large saucepan. Add diced onion and a pinch of salt, and sauté for 5-7 minutes until soft and translucent.

Stir in 4 minced garlic cloves and halved grape tomatoes. Add more salt, cover, and cook for around 25 minutes, stirring occasionally.

After 25 minutes, gently break up the softened tomatoes and mix in reserved hot pasta water until desired consistency is achieved.

Stir in ½ cup of grated Parmesan cheese and chopped basil, allowing to simmer for an additional 5 minutes.

Add the cooked spaghetti to the sauce, gently tossing to coat each strand. Allow to warm for a minute before removing from heat.

Plate the spaghetti, drizzling with extra olive oil, adding additional grated Parmesan and red pepper flakes if desired. Serve immediately.