Heat a tablespoon of olive oil in a heavy-bottomed pot over medium heat until shimmering and fragrant.
Add diced onion, sliced carrots, and chopped celery. Cook, stirring occasionally, until the vegetables soften and release their sweet aroma, about 8 minutes.
Stir in small cubes of chicken breast, season with salt and pepper, and cook until the outside turns opaque, about 5 minutes.
Pour in the chicken broth and add dried oregano and a bay leaf. Bring the mixture to a gentle simmer, just below boiling, around 90°C (194°F).
Let the soup simmer gently for 20-25 minutes, stirring occasionally, until the chicken is cooked through and the vegetables are tender.
Remove the chicken breasts from the pot, shred them with two forks, and return the shredded chicken to the soup.
Squeeze fresh lemon juice from half of the lemon into the soup, stirring to brighten the flavors.
Taste and adjust the seasoning with more salt, pepper, or lemon if desired.
Remove the bay leaf, ladle the hot soup into bowls, and serve immediately, enjoying the comforting warmth and vibrant flavors.
