Put the lentils in saucepan of cold water and bring to the boil. Simmer for 15-20 minutes, until tender, then drain and leave it to cool. Skip this if using tinned lentils.!
Meanwhile peel the avocado, slice its flesh and squeeze some lemon juice over to stop it discolouring.
Depending on the size of the tomatoes, cut them into halves or quarters.
Scatter the salad into a bowl, add the tomatoes, avocado slices and the lentils.
Tear the mackerel into chunks and add to the salad.
Cut the chives cut into 3-4 cm long pieces and scatter on top of the salad.
Mix the lemon juice and oil with 1 pinch of salt and pepper, then drizzle over the salad.