Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, about 9-11 minutes. Taste a minute or two before the end for perfect texture.

Heat a large skillet over medium heat. Add salted butter and let it melt.

Add chopped onions and sauté for about 5 minutes until soft and translucent.

Add chopped garlic to the onions and stir for about 1 minute until fragrant but not browned.

Stir in the tomato paste and cook for another minute until caramelized.

Pour in the crushed tomatoes and add red pepper flakes if you want some spice. Let it simmer for about 10 minutes, stirring occasionally.

Lower the heat and add heavy cream. Stir until well-combined, letting it simmer for about 5 minutes to thicken slightly.

Once the pasta is cooked and drained, add it to the creamy tomato sauce. Toss gently to coat. If the sauce is too thick, add a splash of pasta cooking water.

Stir in the grated Parmesan cheese and half of the fresh basil. Sprinkle the torn mozzarella over the top.

Spoon the pasta into bowls, top with more basil and extra Parmesan as desired, and serve hot.