Place the avocado in a small bowl and mash roughly with a fork. Season well with salt and black pepper.
Place the harissa paste in a small non-stick frying pan and place over a low heat. Heat for about a minute or until just starting to bubble. Spread the harissa out a little in the centre of the frying pan and crack the egg on top.
As the egg starts to cook, gently push any harissa around the edge, into the egg white. Continue to gently cook the egg over a low heat for about 4-5 minutes, or until the white is cooked and the yolk is cooked to your own liking. Take care not to overheat the harissa as it may burn.
While the egg is cooking, toast the slice of bread and spread the mashed avocado over.
When the egg is cooked to your liking, remove it from the frying pan using a fish slice and place on top of the mashed avocado. Sprinkle with a few dried chilli flakes, if wished, and serve at once, either alone or with a crisp mixed side salad or other accompaniment of your choice.