Pour the chicken stock into a large soup pot and bring it to a gentle simmer over medium heat.
While the stock warms, season the chicken breasts with salt and pepper, then place them in a hot skillet. Sear for about 4 minutes per side until golden and cooked through.
Remove the chicken from the skillet, let it rest for 5 minutes, then shred into bite-sized pieces with two forks.
Dice the onion and mince the garlic, then sauté them in the same skillet until translucent and fragrant, about 3-4 minutes.
Add the sautéed onions and garlic to the simmering stock, stirring to combine.
Return the shredded chicken to the pot, then stir in freshly squeezed lemon juice and lemon zest. Let everything simmer together for 10 minutes to meld the flavors.
Taste the broth and adjust with more lemon juice, salt, or pepper as needed for brightness and balance.
For a heartier soup, cook noodles or rice separately according to package instructions, then add to individual bowls before ladling the hot soup over.
Serve the soup hot, garnished with chopped herbs and a lemon slice for an extra zing, enjoying the bright, comforting flavors in every spoonful.