Before chopping portobello mushrooms, use a spoon to scrape away the dark brown gills on the undersides to keep the mushroom spread from turning a dark grey color. Prepare and refrigerate the dip up to a day in advance. Then pop in the microwave for a couple of minutes to reheat. Toast the crostini up to 3 days prior to the party, and store them at room temperature in a zip-top plastic bag. Use truffle oil on the bread, if desired.
Place bread slices in a single layer on a large baking sheet. Brush 1 side of slices evenly with 1 tablespoon oil. Broil 1 minute or until lightly browned.
Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add shallots and garlic; sauté 1 minute. Add mushrooms; sauté 5 minutes or until tender. Add wine, 1/3 cup parsley, thyme, salt, and pepper; bring to a boil. Cook 15 minutes or until liquid almost evaporates. Remove from heat; stir in sour cream and vinegar. Spoon into a serving bowl, and sprinkle with 1 tablespoon parsley.