Heat olive oil in a large deep skillet or Dutch oven over medium-high heat.
Season both sides of the chicken breasts with kosher salt and black pepper.
Add the chicken breasts to the skillet and cook for 5-6 minutes per side until golden brown and cooked through. Remove chicken from the skillet and set aside.
Reduce heat to medium and add the butter to the same skillet.
Once the butter is melted, add the minced garlic and chopped onion. Sauté for 2-3 minutes until fragrant and the onion is translucent.
Add the rinsed quinoa to the skillet and stir to coat the quinoa in the garlic butter mixture for 1 minute.
Pour in the chicken broth and bring the mixture to a boil.
Reduce heat to low, cover the skillet, and simmer for 15 minutes until the quinoa is tender and the liquid is absorbed.
Stir in the chopped baby spinach and Parmesan cheese until the spinach wilts and the cheese is melted.
Return the cooked chicken breasts to the skillet on top of the quinoa mixture. Cover and cook for an additional 2-3 minutes to warm the chicken through.
Sprinkle chopped fresh parsley over the dish before serving.