First cook the egg with salt and lemon juice or vinegar on a low heat. Do not let the water boil. If it begins to boil, reduce the heat.
To poach the egg, break it into a small bowl or cup and pour it into the water.
Cook on a low heat for 3 to 4 minutes until the egg white is solid, but the yolk is still liquid. Remove and place on a paper towel to absorb the excess water.
Mash the avocado with the oil, salt and pepper until it has a puréed texture.
Make the sandwich, putting the green salad or spinach on the slice of bread, then the avocado purée, with the egg on top, completing the sandwich with the other slice of bread.