Preheat the grill.
Pop out the stem from all of your portobello mushrooms.
Season both sides of mushrooms. Start by drizzling olive oil (not too much, just a slight drizzle), some salt and fresh black pepper.
Cook mushrooms over a hot fire for about 4 minutes on each side.
Meanwhile: Split the pieces of bread, and pull out some of the soft center to make a hole for the mushroom burger. Place the bread face down on the grill to toast.
Place the mushroom burger in the bread, and top with a tablespoon of hummus and a chunk of feta cheese right in the middle and cover with a piece of bread.