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Fill a large saucepan with water, add the vinegar and bring to a hard boil over a high heat. Once boiling, reduce the heat so the water is simmering, then use a spoon to swirl the water into a whirlpool. Crack in both eggs and cook for 2 1/2 mins.
Meanwhile, in a small bowl, mash the avocado, feta, chilli flakes and lemon juice, and season well with black pepper. Toast the bread.
Pile the avocado and feta on the toast, then put a poached egg on top of each.