"This is one of my favorite Brussels sprouts dishes and I have literally eaten the whole dish by myself (though I was very, very full afterwards). If you are serving it as a side, it'll feed 5- to 6 people. As a main. I would say 2 or 3, but 4 is probably more reasonable for normal vegetable eaters.... Do not skimp on the salt, pepper, and nutmeg otherwise the casserole will be too bland (but you can of course always salt at the table)."
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until softened, about 10 minutes. Add Brussels sprouts and cook for 5 minutes more. Drain.
Preheat oven to 375 degrees F (190 degrees C). Grease a baking dish.
Beat heavy cream in a chilled glass or metal bowl with an electric mixer until soft peaks form. Beat eggs in a separate bowl until foamy, 2 to 3 minutes, and generously season with salt, pepper, and nutmeg. Stir in Cheddar cheese. Fold in whipped cream.
Place drained potatoes and Brussels sprouts in the prepared baking dish. Cover evenly with egg-cream mixture.
Bake in the preheated oven until top is lightly browned, 25 to 35 minutes.