In a medium pot, add quinoa, water and a pinch of salt; cover and bring to a boil. Reduce heat to low, cook for 12 minutes and set aside.
Preheat large non-stick skillet on medium heat and swirl olive oil to coat. Add onion and garlic, saute until translucent, stirring occasionally.
Preheat oven to 375 degrees F and spray 9" x 13" baking dish with cooking spray. In a small bowl, whisk together milk with cornstarch. Add to a skillet with previously cooked onion and garlic. Also add 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 a prepared baking dish, level and sprinkle with remaining 1/4 cup cheese. Bake uncovered for 25 minutes. Serve hot.