Cream of Portobello Mushroom Soup with Sherry

3/4 pound portobello mushrooms

1 clove garlic, minced

1 small red onion, diced

1/4 teaspoon Montreal Steak Seasoning

1 tablespoon unsalted butter

1 1/2 cups beef stock

1/2 cup heavy cream

1/4 cup medium-dry Sherry

Clean mushrooms, remove gills, and then quarter and thinly slice. In a heavy saucepan cook mushrooms, garlic, and onion in butter over moderate heat, stirring occasionally, until softened and any liquid mushrooms give off is evaporated. Sprinkle with steak seasoning. Add stock and cream and simmer 15 minutes. In a blender or food processor, purée half of mixture. (Alternately, you can use an immersion blender, although the pieces of mushrooms tend to get stuck.) Return to pan and stir in sherry, and salt and pepper to taste. Heat soup until hot and serve.