Preheat oven to 400℉.
In a 10-inch cast iron skillet set over medium-low heat, melt the butter and add the diced onion. Cook for 2-3 minutes, stirring occasionally. Next, add in the minced garlic and cook for 30 seconds.
Add the flour and whisk until combined. Cook for 5 minutes, whisking occasionally. Next, slowly whisk in the milk, in half cup increments, until all of the milk is incorporated. Allow the sauce to simmer over low heat for about 5-10 minutes, or until it has thickened.
Season the Béchamel with the sea salt, black pepper, cayenne pepper, ancho chili powder, and nutmeg.
Stir in the chopped spinach and artichoke hearts. Then stir in the chopped and seasoned shrimp.
Next, stir in the Gruyere cheese, 3/4 of the mozzarella cheese, and 3/4 of the white cheddar cheese. Top the Shrimp Spinach Artichoke Dip with the remaining cheese and bake for 20 minutes.
Remove the Shrimp Spinach Artichoke Dip from the oven and allow to cool for at least 5 minutes before serving. Serve with organic tortilla chips.