Dice the leek or onion finely and set aside.
Heat a large heavy-bottomed pot over medium heat and add olive oil, allowing it to shimmer.
Add the diced leek or onion to the pot and cook, stirring occasionally, until translucent and fragrant, about 5-7 minutes.
Add the minced garlic and cook for another 30 seconds until fragrant, being careful not to let it brown.
Cut the chicken into bite-sized pieces and add to the pot, stirring until lightly browned and cooked through, about 5-8 minutes.
Pour in the chicken broth and bring the mixture to a gentle simmer, then reduce the heat to low and cook for 15 minutes to develop flavor.
While the soup simmers, zest the lemon and squeeze out about 2-3 tablespoons of fresh lemon juice.
Add the lemon zest and juice to the soup, stirring well to combine and brighten the broth.
Taste the soup and season with salt and freshly ground pepper as needed.
Ladle the hot soup into bowls, garnish generously with chopped herbs, and serve immediately.
