In a deep kettle combine the turkey breast with enough cold water to cover it by 1 inch and remove the turkey breast. To the kettle add the onions, the carrots, the bay leaves, the peppercorns, and the vinegar, bring the mixture to a boil, and add salt to taste. Return the turkey breast to the kettle and poach it, covered, for 30 minutes, and drain it, discarding the other solids.
In a skillet cook bacon over moderate heat until crisp and transfer to paper towels to drain. Toast bread and spread 2 slices with blue cheese and remaining 2 slices with mayonnaise. Thinly slice turkey diagonally. Pit and peel avocado and thinly slice. Divide turkey, bacon, avocado, and lettuce between blue-cheese bread slices and top with remaining 2 slices.