Sauté the Onion: Heat olive oil in a large skillet over medium-high heat. Add the diced onion and 1 teaspoon of salt, cooking for about 5 minutes until the onion becomes soft and translucent.

Make the Creamy Base: Pour in the heavy cream, stirring frequently, and let it simmer for 5 minutes until slightly thickened.

Thicken the Sauce: In a small bowl, whisk 1 tablespoon of flour with 3 tablespoons of water until smooth. Gradually stir this mixture into the sauce and cook for an additional 5 minutes, allowing it to thicken further.

Add the Cheeses: Reduce the heat to low and stir in the Parmesan, mozzarella, and garlic powder. Let the sauce simmer for 3-5 minutes, until the cheese is fully melted and incorporated. If desired, add a sprinkle of freshly chopped parsley for extra flavor.

Combine Pasta and Sauce: Add the cooked macaroni to the skillet, tossing to coat each piece in the creamy sauce.

Infuse with Pesto: Remove the skillet from heat and stir in the pesto sauce until evenly mixed.

Serve and Enjoy: Garnish with more parsley or a sprinkle of Parmesan, and serve immediately.