1. In a 10- to 12-inch nonstick frying pan over medium-high heat, stir bacon often until browned and crisp, 3 to 5 minutes. With a slotted spoon, transfer to paper towels to drain. Spoon out and discard all but 1 tablespoon fat from pan.
2. Meanwhile, rinse and drain mushrooms. Trim off and discard stem ends and any bruised areas; thinly slice mushrooms. Add to pan and stir often until browned, about 5 minutes. Add turkey and cooked bacon and stir until hot, 1 to 2 minutes. Remove from heat and stir in vinegar.
3. In a large bowl, combine lettuce, watercress, and onion. Add turkey mixture and mix well. Add salt and pepper to taste. Divide salad equally among four dinner plates. Sprinkle evenly with blue cheese if desired.