Strain the thawed spinach through a colander, and then wrap in a kitchen towel and squeeze out as much liquid as possible.
Melt the butter in a large straight-sided skillet over medium heat. Cook the onions until softened, 6 to 8 minutes, and then season with salt and pepper.
Add the cream and nutmeg and cook until the cream is reduced by half, 2 to 3 minutes. Stir in the cheese until melted, and then mix in the spinach to combine. Cover and cook over medium-low heat until the spinach is tender, 8 to 10 minutes, and then season with salt and pepper. Transfer to a bowl and serve immediately.