Open up the mackerel can and drain it. Put the pieces into a bowl and use a fork to make smaller chunks. Make thin tomatoes of the tomatoes and slice the onions in a way that they resemble half moons. Add the tomato and onions to the mackerel followed by some basil (use half basil, chop up the other half).
Chop down the basil roughly. Mince the garlic clove and put it into a smaller bowl for preparing the dressing. Add the olive oil, balsamic, chopped basil and salt and pepper. Mix well to get an even taste. This will serve as your dressing.
Place lettuce and spinach leaves on salad platters and scoop the mackerel mixture on them. Top it up with the garlic dressing you prepared. Your tomato,basil and mackerel salad is ready. You can also include any vegetables you have in the kitchen. Sprinkling a bit of mozzarella over the salad is only going to make it better.