Cook the spinach according to package directions. When cool enough to handle, squeeze out all the liquid that you can. Set aside.

Melt butter in a 12-inch skillet over medium heat.

Add onion and garlic and cook, stirring occasionally, until somewhat softened, 2 to 3 minutes.

Add flour and cook, stirring constantly, for a minute or so, until foamy.

Pour in milk and stir vigorously to dissolve flour mixture.

Add nutmeg, cayenne, salt, and pepper. Bring to a boil, then reduce heat and simmer for five minutes, until slightly thickened.

Stir in spinach and grated cheese, cooking for a minute more to warm through.