For your pasta, boil some salted water and cook it till al dente as directed on the packet. While it cooks, prepare your other ingredients.
Add the oil to a small pan over medium heat when the pasta is nearly done. Add the garlic to the heated oil and sauté for 30 seconds.
Add the parsley, lemon juice, and tuna by stirring. Let it warm up fully.
When the pasta is finished cooking, drain it and toss it with the sauce after adding a small amount of the pasta water (a few tablespoons) to the sauce. Add salt and pepper to taste.
NOTES
Pasta never really reheats well, especially these varieties of pasta that don't come with a ton of tomato or creamy sauce. Leftovers will keep for 2 days in the fridge. The best way to reheat food in the microwave is with a dash of water (or, even better, save some pasta boiling water!).
To your liking, add more or less red pepper flakes, or go all out and make it hot.
If desired, squeeze some lemon juice in.
If preferred, bake your tuna pasta or prepare it like a casserole. Make sure there is sauce in the finished dish before baking so that it doesn't turn out dry when creating this.
Use gluten-free pasta to convert this dish to a gluten-free version.