Step 1

Heat olive oil in a large pan over medium heat. Add onion and cook until beginning to caramelize, about 10 minutes. Add tuna with oil, capers, and crushed red pepper flakes; heat through.

Step 2

Meanwhile, bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water for 1 minute less than the package instructs, about 10 minutes. Stir occasionally to prevent sticking. Drain, reserving 1/2 cup of the starchy pasta water.

Step 3

Add the cooked pasta to the pan and mix to combine. Add some of the pasta water and toss lightly until the pasta is tender yet firm to the bite, 1 to 2 more minutes. Sprinkle with chopped parsley and serve immediately.