
In a pot, melt 1/2 cup butter. Sprinkle in the flour and whisk together. Cook over medium heat until light golden brown, about 5 minutes. Add the onion and garlic and stir together, cooking for another minute. Pour in the milk, whisking constantly, and cook for another 5 minutes.
Meanwhile, in a separate pot, melt the remaining 3 tablespoons butter. Add the spinach in increments until all is incorporated, and cook until wilted but not soggy, 4 to 5 minutes.
Season the cream sauce with salt, pepper, and nutmeg. Add the spinach to the cream sauce, stirring gently to combine. Serve immediately.