Dice the onion, carrots and celery and mince the garlic. Rinse the quinoa.
Heat the oil in a large pot over medium heat. Add the onion, carrot and celery and cook, stirring, until soft, about 6 minutes. Add the garlic for 30 seconds.
Add the broth and whole chicken breasts. Bring to a boil over high heat, then reduce to low so it simmers gently and cook for 15 minutes, until the chicken reaches 74°C (165°F) in the thickest part.
Lift the chicken onto a board and shred it with two forks.
Add the quinoa to the pot and simmer over medium-low heat for 12 minutes, until the quinoa is tender and shows its little white tails.
Return the chicken and stir in the spinach until it wilts, then add the lemon juice, chopped dill and pepper. Taste and adjust.