Sandra's Spinach Maria

Ingredients

• 6 tbsp butter, separated into 1 tbsp and 5 tbsp
• 1/2 yellow onion, diced
• 4 cups milk
• 2 tsp garlic powder
• 1 tsp crushed red pepper flakes
• 1 tsp mustard powder
• 5 pkgs frozen spinach, thawed (50 oz total)
• 5 tbsp flour
• 1 cup Velveeta cheese, cut into slices
• 1 cup shredded sharp Cheddar cheese
• 1-1/2 cup grated Monterey Jack cheese, divided into 1/2 cup and 1 cup