Slice the Portobello into 1/4-inch slices, then cut each slice into pieces about 2-1/2 inches long. Mix the sherry, 2 tablespoons of oil, salt, and pepper in a glass bowl and marinate the mushroom strips for about 30 minutes.
In a large skillet, heat one tablespoon of olive oil. Wipe off the marinating mushroom pieces and saute in the oil until just tender. Remove to a side dish and reserve.
In the same skillet, heat the remaining oil and butter and saute the onions and garlic until translucent. Add the sliced buttons mushrooms and continue cooking until they start to caramelize, stirring frequently. Season with salt and pepper.
Remove the skillet from the heat and blend in the sour cream. Reheat the mixture over low heat, being careful not to bring it to a boil. Add the sauteed Portobellos and continue to heat over low. Taste for seasonings and adjust. Sprinkle with chopped parsley and serve immediately.