Blanch spinach in boiling water 30–40 seconds, then cool in ice water and drain.
Squeeze out excess water and roughly chop if desired.
Melt butter in a skillet, sauté onions with salt and pepper until soft.
Add garlic and cook 1 minute until fragrant.
Stir in flour and cook 1 minute.
Gradually whisk in half-and-half, season with nutmeg, cayenne, salt, and pepper, and simmer until thickened.
Stir in spinach until evenly coated; thin with milk if needed.
Adjust seasoning and serve warm alongside your main dish.