Spinach Maria

Preheat oven to 350°. In a large saucepan over medium heat, melt 1 tablespoon butter. Add onion and cook until softened and lightly golden, 4 to 5 minutes. Add milk, garlic powder, red pepper flakes and mustard powder to saucepan. Bring the mixture to a boil, stirring regularly. As soon as it starts to boil, reduce heat to simmer, leaving it to thicken. Melt remaining 5 tablespoons butter in a smaller saucepan, then sprinkle in flour and stir to form a paste, cooking over low heat for 2 to 3 minutes. Add mixture to the large saucepan and stir to combine. Reduce heat on the large saucepan to medium, then gradually add Velveeta and cheddar, stirring regularly so it melts, creating a thick sauce. Add in 1/2 cup Monterey Jack cheese. Once all of the cheese has melted, remove 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 to 17 minutes, or until cheese has melted and mixture is bubbling slightly.