Heat olive oil in a large soup pot over medium heat. Add chopped onion and cook until soft and translucent.
Add minced garlic and cook for 30 seconds until fragrant.
Add diced chicken, season with salt and black pepper, and cook for 3–4 minutes until lightly cooked on the outside.
Pour in chicken broth and bring to a gentle simmer. Cook for 12–15 minutes until the chicken is fully cooked and tender.
Stir in fresh spinach and cook for 2–3 minutes until wilted.
Taste and adjust seasoning. Add lemon juice for brightness or cream for richness, if desired.
Serve warm.