1 (7-pound) chicken, rinsed, giblets and neck discarded
1 3/4 sticks (14 tablespoons) butter, plus more for buttering the dish
2 pounds cremini mushrooms, thinly sliced
1 1/2 cups half-and-half
2 cups whole milk
1 medium leek, halved lengthwise, rinsed, and thinly sliced
4 medium garlic cloves, roughly chopped
1/2 cup all-purpose flour
1 teaspoon hot sauce (such as Tabasco)
1/4 cup freshly grated Parmigiano-Reggiano cheese (1/2 ounce)
1 cup grated Emmentaler cheese (2 1/2 ounces)
1 tablespoon chopped fresh thyme leaves
1 (9-ounce) package frozen quartered artichoke hearts, such as Birds Eye brand (about 3 cups), or cooked fresh artichoke hearts
1/2 cup plain dry breadcrumbs
Place chicken broth, salt, fennel, onions, and chicken in a large pot and bring to a boil over high heat. When it boils, reduce heat to low, cover, and simmer until chicken is tender and completely cooked, about 1 hour. Remove chicken from liquid and place on a cooling rack set on a baking sheet to cool. Reserve the cooking liquid for another use.
Melt 1 tablespoon butter in a large frying pan over medium heat. When it foams, add 1/4 of the mushrooms. Cook, stirring occasionally, until mushrooms are golden brown, about 10 minutes. Remove from pan, season well with salt and freshly ground black pepper, and set aside. Cook remaining mushrooms in 3 more batches, using 1 tablespoon of butter per batch.
When the chicken is cool enough to handle, remove skin and cut meat into 1-inch cubes. Discard skin and bones, and reserve meat.
Heat oven to 375°F and arrange rack in middle. Combine half-and-half and milk in a medium saucepan and bring to a simmer over medium-low heat. Once mixture simmers, reduce heat to low to keep warm.
Melt 10 tablespoons of butter over medium heat in a large Dutch oven or heavy-bottomed pot that is wider than it is tall. When it foams, add leek and garlic, and cook until softened, about 2 minutes. Season with salt.
Sift flour over the leek mixture and stir to combine. Cook, stirring constantly, until flour is well incorporated and has lost its raw flavor, about 3 minutes. Remove from heat; slowly add warm half-and-half and milk, stirring constantly until smooth. Return to stove over medium heat and cook sauce until it simmers and begins to thicken, about 3 minutes. Remove from heat and add the hot sauce, Parmiggiano-Reggiano, Emmentaler, and chopped thyme; stir until cheese melts.
Stir in the reserved chicken and mushrooms mixture and the frozen artichoke hearts; taste mixture and adjust seasoning if necessary.
Butter a 13-by-9-inch baking dish. Spread the chicken-and-artichoke mixture in an even layer then evenly sprinkle breadcrumbs over top. Bake, uncovered, until the edges are bubbling, breadcrumbs are browned, and casserole is heated through, about 30 minutes. Allow casserole to sit for 5 minutes before serving.
Beverage pairing:Hiedler Löss Grüner Veltliner, Austria. Artichoke is one of those famous “wine killers,” a food that’s difficult to pair. There’s one wine, by virtue of its green fruits, legume flavors, and pleasing herbaceousness that always seems to stand up: Grüner Veltliner. This is a rich one, but has great acidity and structure to bring definition to the casserole.