When the water is at a full boil, add salt and then wait for it to return to a boil.
Add the pasta and cook per the instructions until al dente (it should be cooked through but still be slightly firm when you take a bite).
1 pound short, thick pasta like cavatappi, fusilli, or penne
Turn off the burner, reserve a cup of pasta water, drain the pasta, and return it to the pasta pot. Add pesto and stir, adding more to your taste. Use a splash of pasta water to create a thin sauce if the pesto is not sticking to your pasta.
Drain and discard the oil from the can of tuna and break it into large or small flakes or chunks, depending on your preference. Mix the tuna gently into the pasta.
Serve warm with additional Parmesan, or let cool to room temperature.
2 cans (10 ounces) tuna packed in oil