In a small saucepan on medium heat, melt butter.
Add onion and garlic and cook until softened.
Add flour and stir with spatula until flour turns golden brown.
Slowly pour in half and half, stirring constantly.
Add nutmeg, cayenne, kosher salt, ground pepper and stir until sauce thickens (coats the back of a spoon).
In a large frying pan on medium heat, place large amount of spinach leaves into pan, cover with a lid to wilt (soften leaves).
Remove from pan.
Wilt another batch of spinach leaves.
When all spinach is wilted, add all spinach and the cream sauce into the large frying pan and stir to combine.