Heat oil in a large pot over medium-high. Add the onions and cook, stirring often, until deeply charred around the edges and tender, 10 to 12 minutes. Add the garlic and cook, stirring often, until softened, 2 to 3 minutes.
Add the chicken, lentils and turmeric, and season generously with salt and lots of pepper. Toss everything a few times so that the turmeric can coat the other ingredients.
Pour in 8 cups water, turn the heat to high and bring to a boil. Then, adjust heat to medium-low and cook, stirring occasionally, until the lentils have split and collapsed into the broth in a golden mush and the chicken is cooked through, 20 to 25 minutes.
Using tongs, remove the chicken thighs, let them cool slightly, then shred and return to the pot. Sprinkle in the herbs, taste and season with salt and pepper as necessary.
In a small bowl, combine the yogurt and lemon juice and season with salt. (If using Greek yogurt, thin with water until it reaches a runny consistency.)
Ladle the soup into bowls, then spoon the lemony yogurt over before serving.
