1. Bring the chicken stock or water to a boil and cook the quinoa until all the water is absorbed. Add in half of the chopped parsley, lemon juice and zest, chopped red onion, and olive oil. Season to taste (this will depend on if your chicken stock was preseasoned, or if water was used in the quinoa). Set the quinoa aside to cool.
2. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
3. Lay the sliced onions on the bottom of a 9- x 13-inch (20- x 30-centimeter) baking dish. Stuff each boneless chicken thigh with about two tablespoons cooled quinoa mixture and roll up, then place on top of the onions.
4. Slice each lemon in half and squeeze over the top of the chicken, then tuck the lemon halves into the pan as well. Sprinkle the chicken with salt, pepper, and garlic powder.
5. Cover tightly and bake for one and a half hours. Sprinkle with remaining parsley before serving.