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.