Heat 2 tablespoons (28g) of olive oil in a Dutch oven or soup pot over medium-high. Once shimmering, add the onion plus ½ teaspoon kosher salt. Cook until onion has softened and is golden brown, stirring occasionally, 10 to 15 minutes. If the onions brown too quickly, add a splash of water and scrape up any browned bits.a. While the onions cook, you can prep some of the other ingredients. Add the carrots, garlic, and preserved lemon peel. Stir frequently until carrots are just starting to soften, about 5 minutes. Add the Aleppo pepper and coriander and stir constantly for 30 seconds. If things are browning quickly or the spices dry out, deglaze with a splash of water and lower the heat a bit.
Pour in a splash of broth and scrape up any browned bits. Add the remaining broth, 1 cup (240 mL) water, lentils, herb bundle, and about 10 twists of black pepper. Stir well, cover, and bring to a boil.
Once boiling, reduce the heat to a simmer. Simmer, covered, for 20 minutes, or until lentils are tender, occasionally uncovering to ensure the soup isn’t bubbling rapidly (the lentils will cook unevenly).
While the soup simmers, make the Herb Oil: Zest the lemon. Finely chop the fresh herbs. Mix herbs and lemon zest together in a small bowl. Stir in 2 tablespoons (28g) good-quality olive oil and season with two pinches of flaky sea salt. Taste, adding more olive oil or salt as desired (I like more olive oil for richness and so it becomes a looser “herb oil”).
While the soup simmers, make the yogurt sauce (optional): In a small bowl, mix the yogurt with 1 tablespoon lemon juice, and season with salt and pepper.
When the lentils are tender, scoop out the bay leaves and thyme; discard. Stir in the nutritional yeast and gently cook, uncovered, for 1 to 2 minutes. Remove from the heat and stir in 1 tablespoon lemon juice. Taste, adding more lemon juice or salt as desired. For a richer, savory flavor, add the extra 1 tablespoon nutritional yeast.
Serve soup in bowls and spoon with the Herb Oil and a dollop of yogurt sauce, if using. Serve with crusty bread.