In a large bowl, combine the spinach, chopped artichokes, spinach,1¼ cups of Monterey Jack cheese, 1¼ cups of Parmesan cheese, cream cheese, mayonnaise, sour cream, milk, heavy cream, garlic, red pepper flakes, salt and pepper, and stir well to combine.
Spoon the mixture into a 1½-quart baking dish. Sprinkle the remaining ¼ cup Monterey Jack and ¼ cup Parmesan cheeses on top. Bake for 25 to 35 minutes, or until browned and bubbly. Serve warm with tortilla or pita chips, crackers, vegetables, or in a bread bowl. Leftovers can be stored in the refrigerator, covered, for up to 4 days.
Notes
The dip can be assembled a day in advance, covered and refrigerated. Bring to room temperature before baking.
You can double the recipe and bake in a 9x13-inch dish or other 3-quart baking dish.
To make in a slow cooker, assemble exactly as directed above, cook on low for 2 hours, then stir together and serve.
You can replace the mayonnaise and sour cream with Greek yogurt.