Melt a knob of butter in a saucepan. Take off the stove and add 1 tablespoon of plain flour. Mix as it gets to a clumpy consistency, and if it's still runny, add another tablespoon of flour.
Keep mixing, and depending on the amount of butter you have melted in the saucepan, you may need another Tablespoon of flour to make the sauce look like a clumpy paste.
Add milk slowly as you mix to eventually make a thick white sauce consistency. Add some grated tasty cheese, bacon bits, onion, garlic, pepper and basically anything you love like prawns, roasted capsicum and mix through.
Take a few large brown swiss/portabella mushrooms and remove their stalks. Place in a pan upside down on a knob of butter (spores facing up/skin down) and cook on high heat until browned on the bottom. Make sure they're not fully cooked and then take them off the burner.
3)Grill the stuffed mushrooms
Spoon the mixture into the mushrooms and top with some dobs of blue cheese, fetta or even smoked cheese - whatever you like. Place them under a hot grill to toast the top, and take care not to over-cook them.