1. Bring vinegar, sugar, and bay leaf to a boil in a small saucepan. Cook, stirring, until sugar dissolves, about 1 minute. Immediately stir in onion and remove from heat; let cool (discard bay leaf).
2. Transfer 3 tablespoons pickling liquid to a bowl. Whisk in mustard and oil; salt and pepper to taste, for dressing.
3. Cook bacon in a large nonstick skillet over moderate heat, turning once, until crisp and golden brown, 6 minutes. Transfer to paper towels to drain; crumble. Transfer 1/4 cup onion (no liquid) to a large bowl and toss with half of bacon, the spinach, turkey, pecans, cheese, and dressing; salt and pepper to taste. Sprinkle with remaining bacon, Drain remaining onion and serve on the side to top salad, if desired. Makes 4 servings.