Chiffonade spinach and place on platter. Mound mushrooms in center of platter. Sprinkle bacon, scallions, and hard boiled egg around mushrooms and spinach. Drizzle with dressing.
In a food processor blend all ingredients except oil until smooth and with motor running add oil in a stream, blending until emulsified. Transfer dressing to a jar and keep chilled, covered, until serving.