Cook quinoa as per package instructions and set aside. Preheat oven to 375 degrees F and spray 9″ x 13″ baking dish with cooking spray.
Preheat large non-stick skillet on medium heat and swirl oil to coat. Add onion and garlic, saute until translucent, stirring occasionally.
In a small bowl, whisk milk with cornstarch and add to the skillet along with artichoke hearts, spinach, Parmesan cheese, 1/2 cup cheddar cheese, quinoa (fluff with a fork a bit before adding), 1/2 tsp salt and pepper. Stir until cheese is melted.
Transfer mixture to prepared baking dish, level and sprinkle with remaining 1/4 cup cheese. Bake uncovered for 25 minutes. Serve hot.