Portobella mushrooms are a type of mushroom.
Maybe you are cooking them too long.
Sautéing sliced mushrooms:
For each eight ounces of sliced mushrooms, melt one tablespoon butter or heat one tablespoon of oil in a large skillet. Add mushrooms. Cook and stir until golden and the released juices have evaporated, about five minutes. Don't overcrowd the skillet or the mushrooms will steam rather than brown.