This one's got rotisserie chicken and white wine. Could it get any better?
Preheat the oven to 375°. In a medium pot, melt the butter over medium-low heat. Whisk in the flour. Cook, whisking constantly, until the flour is absorbed, 1 to 2 minutes. Gradually whisk in the wine, then the milk. Bring the sauce to a simmer, whisking constantly. Fold in the sliced fennel. Let the sauce simmer until thickened, about 5 minutes. Stir in the Parmesan and Dijon; season with salt. Cook until the sauce is smooth, about 5 minutes. Remove from heat.
In a bowl, mix the chicken skin, almonds, oil, and 1/2 tsp. salt.
Spoon half the fennel sauce into a baking dish. Top with the chopped chicken meat and the remaining fennel sauce. Top with the chicken skin. Bake until the gratin is bubbling and the top is light golden brown, 25 to 30 minutes. Garnish with the fennel fronds.