In a large skillet heat 2 Tbsp oil until it shimmers over medium-high heat. Season the chicken and brown on each side for five minutes. Remove from the skillet and set aside.
Add the remaining oil and heat until shimmering. Sauté the onion, celery and carrot until softened, about 3-4 minutes. Add the garlic and cook for 1 minute.
Sprinkle the flour over the veggies and stir into the mixture. Cook the flour for 1 minute then add chicken broth. Whisk to combine. Scrape the bits (the fond) from bottom of the pan. Bring to a boil and reduce the heat to medium-low.
Nestle the chicken evenly throughout the pan. Cover and let simmer for 15 minutes or until the chicken is cooked through and very tender. 5 minutes before serving, stir in the green tomatillo sauce and sprinkle with cilantro and red pepper flakes.
While the chicken is simmering, place the optional garnish ingredients in individual small bowls.
To serve, place chicken and quinoa on a plate and garnish as you like.