After rinsing spinach and removing stems, heat a large saucepan over medium heat. Add enough water to coat the bottom of the pan.
Working in batches, add the spinach and cook, stirring often, until soft and completely wilted, about 3 to 5 minutes. Drain into a colander and repeat this process until all spinach has been cooked. Rinse with cool water. Once cool enough to handle, use your hands to squeeze excess water from the spinach leaves. Place in a clean bowl and set aside.
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.
