Cook bacon in a large skillet until crisp. Drain bacon on paper towels; crumble four of the bacon slices and set aside. Crumble the remaining slice of bacon; cover and chill for garnish.
Remove drippings from pan. Add the olive oil to the pan. Add mushrooms, onion, and garlic. Cook over medium heat until mushrooms are tender. Stir in vinegar, mustard, and pepper. Stir in spinach; cook and stir for 30 seconds or until spinach is wilted. Add cream cheese, stirring until melted. Remove from heat. Stir in sour cream, the 2 tablespoons milk, and the four slices of crumbled bacon. Transfer to a serving bowl. Cover and chill for 4 to 36 hours.
To serve, stir dip. If necessary, stir in additional milk until mixture is of dipping consistency. Top with chilled bacon. Serve with bagel chips or pita crisps. Makes 32 1-tablespoon servings.