Heat a large heavy-bottomed pot over medium heat and add olive oil. Once shimmering, add diced onions and minced garlic, sautéing until fragrant and translucent, about 5 minutes.
Add sliced chicken to the pot and cook until lightly browned, about 7 minutes, stirring occasionally to ensure even cooking.
Pour in chicken broth, then bring the mixture to a gentle simmer. Allow it to cook for about 10 minutes, letting the flavors meld and the chicken stay tender.
While the soup simmers, zest the lemon using a microplane and juice the lemon to extract about 2 tablespoons of fresh lemon juice.
Stir in the lemon zest, lemon juice, chopped parsley, and thyme into the soup. Continue simmering for another 5 minutes to allow the flavors to infuse.
Taste the broth and adjust seasoning with salt and pepper as needed. Remove from heat once flavors are balanced.
Let the soup rest for a couple of minutes to allow flavors to settle before serving.
Serve hot, garnished with extra chopped parsley if desired, enjoying the bright, comforting flavors with tender chicken and a clear, citrusy broth.