Place the large smoked mackerel in a cutting board and remove its head, tail, skin and bones with a sharp knife. Now, cut the flesh of smoked mackerel into small slices.
Note: Make sure to remove any smaller and thinner bones that you see while cutting the flesh into small pieces.
Rinse and drain 4 medium fresh tomatoes. Now, cut them into medium wedges. Remove the external leaves of Belgian endives and cut their heads into half. Discard their hard core and cut them diagonally into fine strips.
Peel a large ripe avocado, cut into half and remove the pip. Now, cut it into thin slices.
Place the smoked mackerel pieces in a large salad bowl. Top with tomato slices, avocado chunks, and Belgian endives. Pour in the olive oil and fresh lemon juice. Sprinkle with some salt and pepper to taste. Stir lightly to mix up the ingredients.
Grab a medium mixing bowl and add in the finely crushed clove garlic. Stir in the mayonnaise, mustard and capres. Continue mixing until it forms smooth sauce.
Place the salad bowl on a large serving tray and place the bowl of sauce on the side. Serve it up and enjoy.