Start by chopping the onion and mincing the garlic, preparing them for sautéing.
Heat a large pot over medium heat and add olive oil. Once shimmering, add the chopped onion and minced garlic, sautéing until fragrant and translucent, about 5 minutes.
Place the chicken breasts or thighs into the pot, browning each side for 3-4 minutes until golden; this builds flavor and keeps the chicken juicy.
Pour in the chicken broth, ensuring it covers the chicken. Bring the mixture to a boil, then reduce heat and simmer gently for 20-25 minutes until the chicken is cooked through and tender.
Remove the chicken from the broth and shred it using two forks. Set aside.
Add sliced zucchini and chopped spinach to the broth, simmering for about 10 minutes until vegetables are tender and bright green.
Return the shredded chicken to the pot, stirring to combine, and cook for an additional 5 minutes to heat through.
Season the soup with salt and pepper to taste, then squeeze fresh lemon juice over each bowl just before serving for brightness.
Garnish with chopped fresh herbs like parsley or cilantro for a fresh finishing touch.
Serve hot, enjoy the comforting, nourishing flavors of this keto chicken soup.