Preheat the oven to 180°C.
Heat the butter in a large saucepan, sauté the onion, mushroom and garlic until soft.
Add the spinach and sauté until wilted and most of the liquid had cooked away.
Remove from the heat and stir the in flour, coating the vegetables.
Place back onto the heat and add the milk, stir continuously until thickened.
Remove from the heat and add the feta, season well.
Pour the sauce generously over the mushrooms and cover each one with a handful of grated cheese (mixed cheddar and parmesan).
Bake in a preheated oven for +- 40 minutes (or until golden and the mushrooms are tender).
For more of BITSOFCAREY's recipes click here