This casserole is great comfort food and serves 6.
Preheat oven to 350 degrees F.
Pat dry chicken with paper towels and season both sides with salt and pepper. Brown the chicken in a medium sauté pan with 2 tablespoons heated olive oil until the chicken is golden brown on the bottom, approx. 4 minutes. Turn the chicken over, reduce the heat and continue cooking until opaque throughout, approx. 10 minutes. Remove the chicken from the pan and transfer to a plate.
In the same pan that the chicken was sautéed, heat 3 tablespoons of olive oil. Add the leek, celery, mushrooms and a pinch of salt, stirring occasionally until the vegetables are golden and tender, approx. 10 minutes. Stir in the flour and cook an additional 2 minutes then add the sherry, stock, milk and bay leaf. Cook until sauce has thickened, approx. 5 minutes. Season with salt and pepper and discard the bay leaf.
Arrange the bread on the bottom of a shallow 2-quart baking dish, overlapping the slices slightly. We prefer the bread to be slightly stale, but not dry, as the sauce will absorb nicely. Spoon half of the sauce and vegetable mixture over the bread. Slice the chicken crosswise into ½ inch pieces and arrange on top of the bread. Top with any accumulated juices from the chicken, the remaining vegetable mixture, the parmesan cheese and parsley. Bake until golden brown and bubbling, approx. 30 minutes. Allow to stand 15 minutes before serving.