Separate dandelion green tops and slice thinly. Cut the stems into half-inch pieces. Heat a large pot over medium heat, add the scallions, carrots, dandelion stems and a pinch of salt. Stir and cook until the carrots are tender and the garlic is soft (about 6-7 minutes). Add a splash of the broth and scrape with a wooden spoon to remove any brown bits (also known as “fond”) stuck to the bottom of the pot. Add chili powder or cumin, tomatoes, lentils and remaining stock. Bring to a boil, then reduce to a simmer, partially covering with a lid. Stir occasionally, until lentils are almost cooked through (about 30 minutes). Uncover completely and cook for another ten minutes or until lentils are soft. Divide among four wide bowls and garnish each with one tablespoon of crème fraîche and dandelion greens.