Place the bone-in chicken thighs in a large heavy-bottomed pot and add enough water to cover. Bring to a gentle boil, then reduce heat and simmer for about 30 minutes, skimming foam as it forms to keep the broth clear.
Remove the chicken thighs from the broth and set aside to cool slightly. Strain the broth through a fine sieve into a clean pot, discarding solids.
Heat olive oil in the same pot over medium heat until shimmering. Add diced onions, carrots, and celery, sautéing until the vegetables are fragrant and slightly softened, about 5 minutes, with the vegetables turning tender but not mushy.
Add minced garlic and thyme to the vegetables, cooking for another minute until fragrant. Pour in the strained chicken broth and bring to a gentle simmer.
Return the broth to a gentle simmer around 90°C (194°F) and cook uncovered for about 1.5 hours, skimming foam as needed to keep the broth clear and flavorful.
Meanwhile, remove the cooled chicken thighs from the bones and shred the meat into bite-sized pieces, discarding any bones and skin.
Once the broth has simmered, add the shredded chicken back into the pot. Stir well and season with salt and pepper to taste.
Let the soup simmer for another 15 minutes to meld the flavors together, filling your kitchen with savory aromas.
Turn off the heat and squeeze fresh lemon juice into the soup, stirring to brighten the rich flavors with citrusy brightness.
Taste and adjust seasoning with additional salt, pepper, or lemon if desired. Let the soup rest off heat for about 5 minutes to allow flavors to settle.
Serve the hot chicken soup in bowls, garnished with fresh herbs if you like, and enjoy its comforting, rustic charm.
