Anchovy-Butter Chicken with Chicken Fat Croutons

(3½ to 4 lb) chicken

kosher salt

freshly ground pepper

unsalted butter (at room temperature)

anchovy fillets (finely chopped)

garlic (finely chopped)

head of garlic (halved crosswise)

bunch fresh thyme , marjoram, oregano and/or rosemary

small red onions or large shallots (unpeeled and quartered)

loaf of crusty bread (torn into 2-inch pieces)

fresh parsley (coarsely chopped)