Set your pasta to cook according to the packet instructions while you cook the sauce. Cook to al dente and drain and set aside if ready before the sauce.
Meanwhile, finely slice the garlic and chop the anchovies.
Warm the oil in a medium skillet/frying pan over a medium heat. Add the garlic and anchovies and cook around 2 - 3 minutes, stirring regularly, until the anchovies start to melt. Take care not to let the garlic burn - you just want it softened and fragrant so reduce the heat if needed.
Add the red pepper flakes and capers, cook a minute more, then add the tomatoes. Increase the heat slightly to bring to a simmer, stirring occasionally, then reduce heat to continue simmering. Cook for around 5 minutes, or slightly longer if using fresh tomatoes, until tomatoes are soft and slightly thickened.
Add the olives, tuna and around half the parsley and mix well. Season with a little salt and pepper, as needed, but you likely won't need much, if any, salt due to the anchovies. Add the drained pasta and toss with tongs so the pasta becomes fairly evenly covered in the sauce. Serve, topped with the remaining parsley.