Wilt the spinach in a large skillet with a splash of water, tossing until just collapsed.
Drain well, then squeeze out excess moisture with a towel. Roughly chop and set aside.
Melt the butter in the same skillet over medium heat. Soften the onion for 3–4 minutes.
Stir in the garlic and cook until fragrant, about 30 seconds.
Sprinkle in the flour and cook, stirring constantly, for 1 minute to form a blond roux.
Gradually whisk in the milk and cream. Simmer, whisking, until slightly thickened, 2–3 minutes.
Add cream cheese and Parmesan; stir until smooth. Season with nutmeg.
Fold in the chopped spinach. Warm through, then season with salt and pepper to taste.