Preheat the oven to 350°F. Spray an 8x8 baking dish (or similar size) with cooking spray, then set it aside.
In a large bowl, blend sour cream and cream cheese until thick and smooth.
Add spinach, Monterey Jack cheese, Parmesan cheese, dried minced onion, garlic, salt, black pepper, and red pepper flakes to the bowl. Stir until thoroughly combined.
Transfer the spinach dip to the prepared baking dish and smooth it into an even layer.
Bake the spinach dip for 10 minutes, or until hot and the cheese is melted.
Serve as desired.