I can't stand the mushroomy flavor of raw mushrooms. Can't take it man, it makes me nauseous. But, luckily for me, I found that balsamic vinegar cuts that flavor like a mentally retarded man with a scythe through some jerk.
Anyway, here's what I do (I suggest doing these two things concurrently):
Slice up some bell peppers (red, yellow, or orange, green isn't ripe) and onions fairly thin, sautee until soft, toss in some balsamic vinegar at the end and stir around until most of the liquid boils away leaving a soggy mess of vegetables.
Remove the stem of your portobello, place on foil-lined baking sheet, fill the upturned mushroom cup with balsamic, rubbing the gills to make them absorb it (they're fairly hydrophobic otherwise). Bake at 350F for a few minutes until visibly soft, about 7 or 8 for me. Bake too long and it'll deflate and get chewy. Pour off excess balsamic if necessary.
Bottom of the bun gets greens, then the mushroom, then the veggies on top, then the top of the bun. Extremely tasty, especially if you like balsamic vinegar.