Note: look for canned chipotle chilies and oyster sauce in the ethnic section of your supermarket. Preheat oven to 400 degrees. Cut stems off portobellos and use for another purpose, such as duxelles. Clean caps with a wet cloth. Bake Mushrooms about 3 minutes ~ longer if you are going to eat them with a fork. Meanwhile puree chipotles and oyster sauce in a blender until smooth; add a little water if necessary. Set aside. Heat oil over medium heat in a large saute pan. Saute onions and poblanos until mixture begins to wilt, then add salt to taste. Mixture should be somewhat moist rather than dry because Onion and chilies can dry out in oven. Continue to saute 3 minutes, then remove from heat and cool. Place portobellos with gill side up in front of you. Brush top with chipotle-oyster sauce