Heat olive oil in a pan, add minced garlic, and sauté until fragrant. Add sliced onion and cook until softened.

Add mackerel (with its juice), cherry tomatoes, water, and season with salt and pepper. Boil on high heat for 3 minutes.

Remove the mackerel and continue simmering the sauce for 4 more minutes. Tear basil leaves and add them before turning off the heat.