Cook the orzo according to the package directions; set aside. In a bowl, combine half the onions, the tomatoes, red pepper, basil, Feta, 2 tablespoons of the oil, and the vinegar, plus salt and pepper to taste; set aside. Heat the remaining oil in a large saucepan over medium heat. Add the remaining onion, the garlic, and chili flakes and cook until the onion softens, about 3 minutes. Add the beans, 3 cups of the broth, and the lemon juice and bring to a boil. Reduce heat to medium-low and simmer for 5 minutes. Remove from heat and set aside to cool slightly. Transfer to a food processor or blender. Pulse until combined but still slightly chunky. Season with salt and pepper to taste. Return to heat. Add the remaining broth and bring to a simmer. Stir in the orzo. Ladle the soup into serving bowls. Spoon the salsa over the top. Serve with crusty bread, if desired.
Tip: Use caution when pulsing the soup. Hot liquids expand.