Rinse the whole chicken under cold water and pat it dry. Place it in a large heavy-bottomed pot.
Heat the pot over medium-high heat and sear the chicken until golden brown on all sides, about 8 minutes per side. Use tongs to carefully turn it for even coloring.
Add chopped onions, sliced carrots, and chopped celery around the chicken. Let them sweat and become fragrant, about 5 minutes, until they soften slightly and release aromas.
Pour in about 2 liters of cold water to cover the chicken and vegetables. Bring the water to a gentle simmer, skimming off any foam that rises to the surface.
Lower the heat, add fresh thyme sprigs and bay leaves, then cover partially. Let the chicken simmer slowly for 1.5 to 2 hours, until the meat is tender and falling off the bone.
Carefully remove the chicken from the broth using tongs and set it aside to cool slightly. Strain the broth through a fine-mesh strainer into another pot to remove solids and impurities.
Once cooled enough to handle, shred the chicken meat, discarding bones and skin. Return the shredded meat to the strained broth.
If using noodles, add them now to the simmering broth and cook for about 10 minutes until tender.
Stir in the lemon zest and juice, then season with salt and pepper to taste. Adjust flavors as needed.
Finish with a squeeze of fresh lemon juice and a sprinkle of herbs if desired. Serve hot in bowls, with a slice of lemon if you like extra brightness.
