Heat a large frying pan over a medium heat. Butter both sides of each slice of bread, then cut a large heart (or circle) out of the centre of one slice using a knife or biscuit cutter. Place the slices of bread, including the cut-out piece, into the frying pan and cook for 2 mins until golden. While the bread is cooking, slice the avocado and squeeze over the lemon juice. Toss the watercress in a little lemon juice, too.
Flip the bread over, then crack the egg into the cut-out hole. After 1-2 mins, the whole slice of bread and cut-out piece should be golden brown on both sides. Transfer to a plate (eat the cut-out as a chef’s perk, if you like!). Cover the pan with a lid, or some foil, and lower the heat. Continue cooking for a further 1-2 mins until the egg is done to your liking.
Top the toasted slice of bread with the avocado, smoked salmon and watercress, and season with a twist of pepper. Put the egg-in-the-hole slice on top and serve with an extra wedge of lemon, if you like.