Heat olive oil in a large pot or deep skillet over medium-high heat.
Add the chicken pieces and cook for 5-6 minutes, stirring occasionally, until browned and cooked through. Remove the chicken from the pot and set aside.
In the same pot, add the diced onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the pot and cook for another 4-5 minutes until the mushrooms have released their moisture and are tender.
Add the rinsed quinoa to the pot and stir to combine with the vegetables.
Pour in the chicken broth, then add dried thyme, dried oregano, black pepper, and salt. Stir well.
Bring the mixture to a boil, then reduce heat to low. Cover the pot and simmer for 15 minutes, or until the quinoa has absorbed the liquid and is tender.
Return the cooked chicken to the pot and stir to combine.
Add the fresh spinach leaves on top, cover the pot, and cook for 2-3 minutes until the spinach is wilted.
Gently stir the spinach into the quinoa and chicken mixture.
Remove from heat and let sit covered for 2 minutes before serving.