In a microwave-safe bowl, add sweet potatoes or squash, onions, and red bell peppers. Cook in microwave for 4 minutes, to soften.
Heat oil in a large nonstick pot or pan. Add sweet potatoes, onions, peppers, and garlic and cook over medium-high heat, stirring often, for 2 minutes. Stir in chili powder, cumin, and red pepper flakes; reduce heat to medium. Cook, covered, for 4-5 minutes or until sweet potatoes are tender, stirring occasionally.
Add broth, beans, tomatoes, and corn. Bring to boiling, stirring occasionally. Reduce heat. Simmer, uncovered, for 15 minutes. Stir in lime or lemon juice.
To Serve: Ladle 1½ cups into each bowl. Top each with 1 tablespoon cilantro and yogurt, if desired.