In a 10- to 12-inch frying pan over medium-high heat, stir bacon often until browned and crisp, 10 to 15 minutes; spoon out and discard fat in pan as it accumulates. With a slotted spoon, transfer bacon to towels to drain; discard remaining fat in pan.
In a 1-cup glass measure or small bowl, mix olive oil, vinegar, mustard, shallot, 1/2 teaspoon pepper, and 1/4 teaspoon salt.
Set aside 4 to 6 watercress sprigs; coarsely chop remaining sprigs. In a large bowl, combine chopped watercress and lettuce. Add all but 2 tablespoons dressing and mix gently to coat.
Arrange equal portions of lettuce mixture in wide, shallow bowls. On each, in pie-shaped wedges, arrange equal portions of bacon, tomatoes, chicken, blue cheese, eggs, and avocado.
Spoon remaining dressing evenly over toppings. Garnish salads with reserved watercress sprigs. Add salt and pepper to taste.