Start by cutting your chicken breasts into uniform bite-sized pieces. Rinse the quinoa thoroughly in a fine-mesh strainer under cold water. This step is important to remove the natural coating called saponin, which can make quinoa taste bitter if left on.
Heat the olive oil in a large skillet or saucepan over medium-high heat. Add the chicken pieces and season them with half of the salt and pepper. Cook for about 4-5 minutes until the chicken is browned on all sides. Stir in the minced garlic and cook for another 60 seconds until it becomes fragrant.
Add the rinsed quinoa, chicken broth, dried oregano, and the remaining salt and pepper to the skillet. Give everything a good stir and bring the liquid to a boil. Once boiling, reduce the heat to low, cover the pan with a tight-fitting lid, and simmer for 15 minutes.
After 15 minutes, check if the liquid is absorbed. If you are using baby spinach, place it on top of the mixture, put the lid back on, and let it sit for 2-3 minutes with the heat off. The residual steam will perfectly wilt the spinach without overcooking it.
Remove the lid and use a fork to gently fluff the quinoa and mix the chicken and greens together. Taste and add a squeeze of fresh lemon juice if desired. Garnish with chopped parsley before serving.