Sauté the vegetables. Heat olive oil in a large Dutch oven or soup pot over medium heat. Add the diced onion and shredded carrots. Cook for 5–6 minutes, stirring occasionally, until softened.
Add the garlic and spices. Stir in the garlic, cumin, curry powder, thyme and crushed red pepper flakes, if using. Cook for 30–60 seconds, stirring constantly, until fragrant.
Add the tomatoes. Stir in the diced tomatoes and cook for 2–3 minutes, allowing the tomatoes to soften and the flavors to develop.
Build the soup. Pour in 3 cups chicken broth, stirring to combine. Bring to a gentle simmer and cook for 5–7 minutes.
Add the lentils. Break apart the steamed lentils and stir them into the soup. Simmer for another 5 minutes, allowing the lentils to absorb the flavors.
Thicken the broth, if desired. Use an immersion blender to briefly blend a small portion of the soup, leaving most of the lentils and vegetables intact. Alternatively, mash some lentils against the side of the pot.
Add the kale. Stir in the chopped kale and simmer for 3–5 minutes, until tender.
Finish with lemon. Remove from the heat and stir in 1 tablespoon fresh lemon juice. Taste and add more lemon, salt and black pepper as needed.
Serve. Ladle into bowls and serve warm, preferably with crusty bread.