Bring a large pot of salted water to a boil. Cook the pasta according to the package directions.
Scoop out a cup of the starchy pasta water and set aside. Drain the pasta in a colander.
In the pot, stir together the pesto, lemon juice, red pepper flakes, and capers.
Add the pasta and tuna and gently toss to coat with the pesto sauce. Use the reserved pasta water, a small amount at a time, to thin the sauce if needed to you preferred consistency.
Garnish with chopped basil and serve with lemon wedges.
Notes
Reserve some starchy pasta water. The water that the pasta cooks in contains lots of starch. It's ideal for thinning the sauce without making it watery. So don't forget to scoop some of that pasta water before draining. I have forgotten this step so many times in the past, so now I set a timer for a minute before the pasta is cooked.
Check for seasoning. Taste the pesto pasta before serving. I haven't included salt and pepper in this recipe, because they are already in the pesto sauce. However, you may want to add them after tasting.
Vegetables. I often add some veggies to this dish. Some of my favorites are halved cherry tomatoes, artichoke hearts, sliced black or green olives, mushrooms, sundried tomatoes, or cooked cauliflower florets. You can also add baby spinach leaves or chopped kale.
Storage. Keep leftover tuna pasta in an airtight container in the fridge for up to two days. You can enjoy it cold as a tuna pasta salad, or gently reheat it in the microwave.