Wipe the mushrooms and place, gill side up, in a greased baking dish.
Place in a preheated 220*C oven for about 8 minutes.
Heat the olive oil in a frying pan and saute the onions until soft. Stir in the sundried tomatoes, olives, and chopped spinach. Cook for a few minutes, then add the chopped plum tomatoes and the crushed croutons. Take off the heat and let sit for a few minutes. Crumble in two thirds of the feta cheese.
Take mushrooms out of oven. Spoon the topping evenly over each mushroom. Crumble the remaining feta cheese over top.
Return to oven and bake until bubbly and feta on top is slightly browned.
Serves 3 as a main course or 6 as a starter, depending on how big your mushrooms are.