- 6 tbsp. butter, separated into 1 tbsp. and 5 tbsp.
- 1/2 yellow onion, diced
- 4 c. milk
- 2 tsp. garlic powder
- 1 tsp. crushed red pepper flakes
- 1 tsp. mustard powder
- 5 packages frozen spinach, thawed (50 ounces total)
- 5 tbsp. flour
- 1 c. Velveeta cheese, cut into slices
- 1 c. shredded sharp Cheddar cheese
- 1 1/2 c. grated Monterey Jack cheese, divided into 1/2 c. and 1 c.
Preheat the oven to 350°F.
Place 1 tbsp. butter in a large saucepan over medium heat. Once it's melted, add the onion and cook until the onion has softened and turned lightly golden, about 4-5 minutes.
Add milk, garlic powder, red pepper flakes and mustard powder into the saucepan. Bring the mixture to a boil, stirring regularly. As soon as it starts to boil, reduce the heat to simmer, leaving it there to thicken.
Melt the remaining 5 tbsp. butter in a smaller saucepan, then sprinkle 5 tbsp. flour in it. Stir the flour in to form a paste, cooking over low heat for about 2-3 minutes. Add this mixture to the large saucepan filled with milk and spices. Stir to combine.
Reduce the heat on the large saucepan to medium, then gradually add the Velveeta and cheddar cheese, stirring regularly so that the cheese melts, creating a thick sauce. Add in 1/2 c. Monterey Jack cheese, setting aside the other cup for topping the spinach dish. Once all of the cheese has melted, remove the saucepan from heat.
Drain any excess water from the thawed spinach. Add it to the cheese sauce, mixing to combine. Pour the mixture into a large casserole dish and top with remaining Monterey Jack cheese. Cook in the oven for 15-17 minutes, or until the cheese has melted and the mixture is bubbly slightly.