Thinly slice the fennel, then roughly chop the tomatoes.
Heat a small spoonful of oil in a frying pan, add the fennel and marjoram. Put a lid on and gently sauté it for 5 minutes. Add the tomatoes, lemon juice and zest. Season to taste.
Put the lid on and gently simmer for about 8-10 minutes or until the vegetables have softened.
Stir in the fresh herbs.
Cut the avocado into thin slices (if used) and place the fish on top (remove the skin of the mackerel, if not liked).
Serve the fish with the fennel & avocado on the side.