Heat oven to 400°F. In skillet or microwave, cook bacon until crisp; drain on paper towels. Crumble bacon; set aside.
Meanwhile, line cookie sheet with heavy-duty foil. Remove pie crust from pouch; place crust flat between 2 sheets of waxed paper. With rolling pin, roll crust into 14x12-inch oval. Remove top waxed paper; carefully turn pie crust over onto cookie sheet. Remove waxed paper.
Spread 2 tablespoons of the mayonnaise evenly over crust to within 2 inches of edge. Sprinkle Monterey Jack cheese over mayonnaise.
In small bowl, mix chicken, remaining 2 tablespoons mayonnaise, the mustard, lemon juice, black pepper, celery salt, salt and ground red pepper. Spoon mixture over Monterey Jack cheese. Sprinkle with crumbled bacon. Sprinkle Cheddar cheese over top.
Fold 2-inch edge of crust up over filling, pleating crust as necessary and leaving 5x7-inch oval opening in center. Gently press down on edge to securely enclose filling.
In small bowl, beat egg yolk and water with fork. Generously brush yolk mixture over crust; sprinkle crust with sesame seed.
Bake 20 to 25 minutes or until crust is golden brown. Cool 10 minutes. With metal spatula, gently lift edge; slide onto serving plate. Sprinkle lettuce over filling; press down lightly. Top with tomato and onions. Cut into 6 wedges to serve.