Prepare the spinach: If using fresh spinach, blanch it in hot water, then drain and set aside (using an ice bath helps preserve the color). For frozen spinach, thaw and drain excess water.

Preheat the oven (optional): If you’re baking the creamed spinach, preheat the oven to 350°F (175°C).

Sauté the onions: In a cast-iron or oven-safe skillet, melt the butter over medium heat. Add the diced onions and cook for about 3-4 minutes until softened and translucent.

Make the cream sauce: Add the heavy cream to the skillet, stirring to combine. Once it begins to bubble, reduce heat to low and stir in Dijon mustard, garlic powder, and red pepper flakes. Simmer for 1-2 minutes.

Add the cheese: Stir in Mozzarella and 4 oz of Cheddar Jack cheese, whisking until a thick cheese sauce forms.

Add the spinach: Squeeze out any excess water from the spinach and stir it into the cheese sauce. Mix well until fully combined.

Optional baking step: If you’re baking the dish, sprinkle the remaining 2 oz of Cheddar Jack cheese over the spinach mixture and bake for 3-4 minutes, or until the top is melted and bubbly.

Garnish and serve: Remove from heat and garnish with the remaining red pepper flakes and black pepper. Serve immediately, hot and delicious!