In a large skillet, add two tablespoons of olive oil and sear chicken breasts on both sides until fully cooked to 165°F. Transfer the chicken to a plate and cover with foil.
Add the remaining tablespoon of olive oil and chopped onion to the skillet, cooking until lightly browned.
Add the chicken stock and quinoa, stirring to deglaze the pan and scrape up the browned bits. Bring to a boil. Reduce heat to low and cover. Simmer for 15 to 20 minutes (stirring occasionally), until quinoa has fully absorbed liquid.
Remove from heat and fluff the quinoa with a fork. Slice the chicken breasts and add it to the pan with the spinach, sun-dried tomatoes, and feta. Add salt and pepper to taste and serve warm.
Notes
Store in an air-tight container in the refrigerator for up to 5 days - makes this great for meal prep!