Separate sardines from their canned juice.

Cut the potato into bite-sized pieces and soak in water. Slice the onion into wedges.

Heat olive oil in a pan, add onion and garlic, and sauté over medium heat until the onion becomes translucent. Add the potato and stir until coated with oil.

Add the canned juice and water, bring to a boil over medium heat, cover, and simmer on low heat for 7 minutes. Add the remaining tomato sauce ingredients, bring to a boil, cover, and simmer for another 5 minutes.