Lemony Black Lentil Soup with Crispy Kale and Yogurt

Name: Lemony Black Lentil Soup with Crispy Kale and Yogurt
Ingredients: 1 medium bunch curly kale (8 to 10 ounces); 1 medium yellow onion; 4 cloves garlic; 1 small lemon; 2 cups dried black lentils (about 1 pound); 4 tablespoons olive oil, divided, plus more for serving; 1 1/4 teaspoons kosher salt, divided, plus more as needed; 1 teaspoon ground cumin; 1/2 teaspoon ground coriander; 1/4 teaspoon red pepper flakes, plus more for serving; 6 cups low-sodium chicken or vegetable broth; 1/2 cup whole or 2% plain Greek yogurt