Preheat the oven to 400 degrees F and butter a shallow 3-quart casserole dish. Bring a large saucepan of salted water to a boil. Add the broccoli and cook until bright green and almost tender (you still want it to have a little bite; it will cook more in the oven). Drain; cool under cold running water and pat very dry. Set aside.
Melt 4 tablespoons butter in a large saucepan over medium heat. When the butter is melted, add the sage and let sizzle a minute, then add the flour. Cook the roux (or else it will taste like raw flour), whisking, for 2 to 3 minutes, without letting it brown. Pour in the sherry (or broth) and cook to reduce it away, about 1 minute.
Whisk in 1 cup broth, the milk, heavy cream and nutmeg and season with salt and pepper. Bring to a simmer and cook, whisking occasionally to get out any lumps, until thickened, 6 to 7 minutes. Stir in the scallions and gruyere until the cheese is melted, then remove the sauce from the heat.
In a large bowl, mix together the sauce, broccoli, chicken and almonds until thoroughly coated. Pour into the prepared casserole dish. In a small bowl, melt the remaining 1 tablespoon butter in the microwave, then toss with the parmesan and breadcrumbs. Sprinkle the mixture over the top of the casserole. Bake until browned and bubbling, about 35 minutes.
Photograph by Ryan Dausch