Clean the portabellas really well, their very dirty. Brush the tops and bottoms with the olive oil generously. Sprinkle salt, pepper, garlic powder, or whatever you would like on them. Toss them into a pan on medium heat, flipping every so often to avoid burning. Meanwhile slice the onion in rings and toss them into the pan with the mushrooms. Put your buns in the toaster. When the mushrooms have started to let out their moister, they are done. Put them and the onions on a bun, with whatever you like on your "burgers".
These are extremely juicy and taste so good, definitely not like a plain ole mushroom if thats what your thinking.
Don't knock it until you try it :)