Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions for al dente (firm to the bite). Reserve about ¼ cup of the pasta water before draining.

Sauté the aromatics: While the pasta cooks, heat olive oil in a pan over medium heat. Add the garlic and onion and cook until softened and fragrant, about 2 minutes.

Flake and heat the tuna: Add the drained tuna to the pan and break it up with a fork. Cook for an additional minute to warm through.

Create the creamy sauce: Add a dollop of Boursin cheese and a splash of heavy whipping cream to the pan. Stir until smooth and creamy. Season with salt and pepper to taste.

Combine pasta and creamy sauce: Drain the pasta, reserving a little bit of the starchy pasta water (about ¼ cup). Toss the cooked pasta with the creamy sauce in the pan. Add a splash or two of reserved pasta water if the sauce seems too thick.

Incorporate the pesto (last minute!): Once the pasta is coated in the creamy sauce, immediately turn off the heat. Stir in your favorite pesto, ensuring it coats everything evenly. Don't cook the pesto here to preserve its vibrant color and fresh flavor.

Serve and enjoy!: Plate the pasta and garnish with grated Parmesan cheese or fresh herbs, if desired. Dive in and enjoy this delicious and flavorful dish!