Add the half-and-half and cream to a small saucepan and heat over medium heat until just starting to simmer. Keep on low, don't bring to a boil.
Using the same skillet that you cooked the spinach in, heat the butter over medium heat. Add the chopped shallots and garlic and cook, stirring occasionally, until soft and translucent, about 3 minutes.
Stir in the flour and cook for another minute.
Whisk in the warmed cream and stir until lumps have dissolved. Cook until thickened, about 3 to 5 minutes more.
Stir in the salt, pepper, nutmeg, and Parmesan cheese. Cook for about another minute.
Stir in the spinach and continue stirring until fully incorporated and the spinach is heated through, about another 3 minutes.
Transfer to a serving dish and serve at once.