Preheat oven to 450 degrees F. Place cauliflower and onion in a shallow baking pan. Drizzle with olive oil and sprinkle with 1/2 tsp. salt. Stir to coat. Spread in an even layer. Roast, uncovered, for 30 minutes or until cauliflower and onion are tender, stirring in cranberries halfway through roasting.
Meanwhile, in a small saucepan whisk together balsamic vinegar, honey, the remaining 1/2 tsp. salt, and pepper. Simmer, uncovered, until sightly thickened, about 10 minutes. Pour balsamic mixture over roasted cauliflower mixture; toss to coat. Transfer mixture to a platter. Sprinkle with mint before serving.