Think macaroni and cheese — only ten times better. This sophisticated version combines the nuttiness of Gruyère with the intense flavor of parmesan, juicy poached chicken, and a crisp, cheesy crust. If you're short on time, feel free to use cooked rotisserie chickens (see cooks' note, below). In that case, though, don't add salt to the sauce, as the stock made from the rotisserie chickens will already be salty.
If using rotisserie chickens, pull meat from bones and reserve, then make stock by cooking skin and bones with celery, carrot, onion, garlic, thyme, salt, peppercorns, and 8 cups water 40 minutes total before straining.
Chicken can be poached and stock can be made 1 day ahead and cooled completely, uncovered, then chilled separately, covered.
Bread crumbs can be made 1 day ahead and kept in a sealed plastic bag at room temperature.
Cheeses can be grated 1 day ahead and chilled separately in sealed plastic bags.
