Dice the onion, celery, and carrots into uniform sizes (this ensures even cooking), mince the garlic, and rinse the lentils under cold water. Heat the olive oil in a large pot or Dutch oven over medium heat. Once shimmering, add the diced onion, celery, and carrots, sautéing for 3-4 minutes until the vegetables begin to soften and the onion becomes translucent. Add the minced garlic, dried oregano, ground cumin, and red pepper flakes, stirring constantly for about 30 seconds to bloom the spices and release their aromatics—this is the foundation of the soup's flavor.
Pour in the chicken broth and crushed tomatoes, scraping the bottom of the pot to release any browned bits. Add the rinsed lentils, bay leaves, and salt, stirring well to combine. Season the chicken thighs with a pinch of salt and pepper, then nestle them into the liquid. I like to leave the chicken thighs whole at this stage so they stay moist during cooking—the larger surface area allows the meat to infuse more flavor into the broth.
Bring the soup to a boil over medium-high heat, then immediately reduce the heat to medium-low and simmer gently for 35-40 minutes. The lentils should become tender but not fall apart, and the chicken should be cooked through and easily shredded. To check doneness, pierce a chicken thigh with a fork—the meat should be very tender and the juices should run clear.
Using a slotted spoon, transfer the cooked chicken thighs to a cutting board or shallow bowl. Let them cool for 1-2 minutes until they're cool enough to handle, then using two forks, shred the chicken into bite-sized pieces, discarding any fat or cartilage. Return the shredded chicken to the pot, stirring gently to distribute it evenly throughout the soup.
Remove the pot from heat and stir in the fresh baby spinach, pushing it down into the warm broth so it begins to wilt. Add the balsamic vinegar or lemon juice, which brightens the soup and balances the earthy lentils and tomatoes. Taste and adjust the seasoning with additional salt and pepper as needed. I find that balsamic vinegar adds a subtle sweetness that complements the Mediterranean flavors, but lemon juice works beautifully too for a sharper, fresher finish.
Ladle the soup into bowls and serve hot. If desired, drizzle a small amount of extra virgin olive oil on top of each bowl and finish with a crack of fresh black pepper for a traditional Mediterranean touch.