Heat olive oil in a large deep skillet or pot over medium-high heat.
Add chicken thighs to the skillet and cook for 5-6 minutes on each side until golden brown and cooked through. Remove chicken from the skillet and set aside.
Reduce heat to medium and add butter to the same skillet. Once melted, add minced garlic and chopped onion. Sauté for 2-3 minutes until fragrant and onions are translucent.
Add rinsed quinoa to the skillet and stir to coat with the garlic butter mixture.
Pour in chicken broth, then add dried Italian seasoning, salt, black pepper, and crushed red pepper flakes. Stir to combine.
Return the cooked chicken thighs to the skillet, placing them on top of the quinoa mixture.
Bring the mixture to a boil, then reduce heat to low. Cover the skillet with a lid and simmer for 15 minutes, or until the quinoa is cooked and liquid is absorbed.
Remove the lid and gently stir in chopped spinach and halved cherry tomatoes. Cook uncovered for an additional 2 minutes until spinach wilts and tomatoes soften slightly.
Sprinkle fresh parsley over the dish before serving.