Heat the oil in a large pan over moderate heat. Add the garlic and onions, and let them simmer for about 2 minutes or until the onions are translucent and aromatic; stir if needed to prevent the garlic from burning.
Add the tomatoes, olives, artichoke hearts, capers, and roasted red pepper. Bring the heat the medium-low, cover, and let simmer for about 5-7 minutes, stirring occasionally, until most of the tomatoes shrivel and/or burst.
Once most of the tomatoes have burst, add the contents of the can of mackerel to the pan and, with a big spoon or spatula, break the mackerel down into chunks or flakes.
Add the water to the pan, followed by the fresh pasta. Stir the pasta into the liquid so it's covered and let it simmer for about 5-7 minutes, or until the pasta is cooked to your preference and the water is absorbed. Finally, add the marinara and toss to coat.
Garnish with fresh parsley and serve.