Heat 1 tablespoon of the oil in a large nonstick skillet over medium-high heat. Season chicken with 1/4 teaspoon of the salt. Cook for 7 to 8 minutes. Remove to a plate.
Add remaining tablespoon of oil, sweet potatoes and onion to skillet and stir-fry for 5 minutes. Add cauliflower and cook 3 minutes. Stir in curry powder, cayenne and the remaining 1/4 teaspoon salt; cook 1 minute. Add broth and tomatoes; cook 5 minutes, covered, stirring occasionally.
Add chicken and heat through. Stir in mint and serve over cooked quinoa.