I got the idea for this sandwhich from Aggie's Kitchen but ended up just doing it my own way instead. So here is my version
Large Portobella Mushroom Caps
Red Pepper (sliced)
Red Onion (sliced)
Salt & Pepper
Coat vegetables with olive oil and sprinkle them with seasoning. Grill mushrooms, onions, and red peppers until done. Remove from grill. Grill buns remove from heat and add slices of brie to the warm bun. Layer the onions and red peppers on top of the mushroom and add to bun. Serve warm.