Bring a large pot of generously salted water to a boil. Add the spaghetti and cook until al dente, about 8-10 minutes. Reserve 1/4 cup of the pasta water, then drain.

While the pasta cooks, heat the olive oil in a large skillet over medium heat. Add the minced garlic and cook, stirring, until fragrant and pale golden, about 1 minute.

Add the drained tuna and pesto to the skillet. Break the tuna into chunky flakes and stir until everything is warmed through, about 2 minutes.

Add the drained spaghetti and reserved pasta water to the skillet. Toss over medium heat for about 1 minute until the pasta is evenly coated and the sauce looks glossy.

Twirl the pasta into bowls. Top with grated Parmesan, a crack of black pepper, and a generous squeeze of lemon. Serve immediately.