Brush or peel the mushrooms and place in a baking tray. Using a pastry brush, brush them with olive oil and sprinkle with a pinch of sea salt. Cover with foil and bake for 20 minutes.
While the mushrooms are baking, cook the broad beans by boiling in water for three minutes. Drain the water and leave to cool before removing the beans from their tougher (and often unpleasant-tasting outer shell). Mix the mint, broad beans, semi-dried tomatoes and feta in a small bowl.
Remove the mushrooms from the oven, fill each with a spoonful of the bean mixture and return to the oven for a further 5 minutes.