Melt 1 stick of butter in a pot. Sprinkle in flour and whisk together. Cook over medium heat for five minutes or until light golden brown. Throw in onion and garlic and stir together, cooking for another minute. Pour in milk, whisking constantly, and cook for another five minutes while you cook the spinach. To cook spinach, melt 3 tablespoons butter in a separate pot. Add spinach in increments until all incorporated, and cook until wilted but not soggy, about 4 to 5 minutes. Season the cream sauce with salt, pepper, and nutmeg. Add spinach to the cream sauce, stirring gently to combine. Serve immediately.