Toasted Grain Soup with Crispy Mushrooms and Kale

Name: Toasted Grain Soup with Crispy Mushrooms and Kale
Ingredients: 1 cup whole grains, such as farro, wheat berries, spelt, barley, or brown rice; 1 pound fresh shiitake mushrooms; 1 bunch curly kale (about 12 ounces); 1 small yellow onion; 4 cloves garlic; 6 tablespoons olive oil, divided, plus more for serving; 2 1/2 teaspoons kosher salt, divided, plus more as needed; Freshly ground black pepper; 1 tablespoon tomato paste; 1/4 teaspoon red pepper flakes, plus more for serving; 7 to 8 cups low-sodium vegetable broth, divided; 1 (15.5-ounce) can chickpeas; 2 teaspoons red wine vinegar; Plain 2% or full-fat Greek yogurt, for serving (optional)