Toast the bread until deeply golden and crisp, so it can hold the avocado without going soggy.
Halve the avocado, remove the pit and scoop the flesh into a bowl. Add the lemon juice and salt and mash with a fork, leaving some texture.
Spread the avocado thickly over the warm toast.
Egg version: fry the eggs in a little olive oil until the whites are set and the edges crisp, about 3 minutes, and slide one onto each toast with a pinch of pepper.
Tomato version: halve the cherry tomatoes, season with a pinch of salt, and pile them on with the feta and a drizzle of oil.
Bean version: toss the beans with a little oil, lemon zest and chili flakes, and spoon them over the avocado.
Finish any version with a drizzle of olive oil and a little extra salt and pepper.