Preheat the oven to 425 degrees.

Butter a 1.5– to 2-quart baking dish and set aside.

In a microwave-safe bowl, cook the spinach until hot.

Place a fine mesh sieve over a large measuring cup or bowl.

Pour the spinach and its liquid into the sieve.

Squeeze the spinach, pressing as much liquid out as possible into the bowl.

Reserve ⅓- ½ cup of the liquid and set it aside.

Make the roux.

In a large saute pan over medium heat, melt the butter and add the minced onions.

Cook until softened – about 5 minutes.

Sprinkle in the flour and stir it into the butter and onions.

Cook, stirring often, until the flour is light browned and smells like baked pie crust – 8 – 9 minutes.

Gradually whisk in the evaporated milk and reserved spinach liquid.

Whisk until the mixture begins to thicken, and the roux is incorporated into the sauce.

Bring the sauce to a low simmer and reduce the heat to low.

Stir in the drained chiles, Worcestershire sauce, garlic salt, celery salt, black pepper, nutmeg, and cayenne.

Add the spinach and 1.5 cups of the grated cheese.

Stir until the cheese has melted completely and the spinach is fully incorporated into the sauce.

Spoon the mixture into the prepared baking dish and sprinkle the remaining cheese over the top.

Bake for 20 minutes or until the top is lightly browned and the mixture is bubbling.

Serve hot.