Cook bacon until crisp, reserving bacon fat. Crumble bacon.
Combine all dressing ingredients in a food processor or small blender at least 30 minutes in advance to allow flavors to combine. Blend until fairly smooth. Adjust seasoning if necessary and hold at room temperature. Stir the dressing before serving and adjust the seasoning again if necessary.
To serve, toss spinach with light coating of dressing Season with salt and pepper if desired. Arrange leaves on plate and garnish with bacon, egg, mushrooms, onion and tomatoes.