Place a medium saucepan onto a medium/high heat and bring to the boil, add the vinegar.
Clean the mushrooms and add butter to a medium frying pan, cook the mushrooms on both side, until golden on both sides. Keep warm until your eggs are ready.
Crack 1 egg into a small ramekin, lower the heat of the water to just simmering,
Slowly pour the egg into the water, crack the second egg into the ramekin and pour that into the water on the other side of the saucepan, allow them to cook for 3-4 minutes for soft yolk eggs, cook longer if you prefer a hard yolk.
Remove the eggs to some kitchen paper to drain off the excess water, place the mushrooms onto a warm plate and crumble the feta cheese over the top of the mushrooms, slice and add the tomatoes, then top with the poached eggs.