Cook 1 sliced shallot in 3 tablespoons olive oil in a large skillet over medium heat until softened, 4 minutes. Add 3 tablespoons capers; cook 1 minute. Add one 14-ounce can cherry tomatoes; cook, stirring, until the tomatoes burst, 5 minutes. Smash with a spatula, then stir in one 7-ounce can Italian oil-packed tuna, breaking it into chunks. Toss with 12 ounces cooked spaghetti and 3 tablespoons olive oil, adding up to 3/4 cup pasta-cooking water to loosen the sauce; season with salt and pepper.