Preheat oven to 375 degrees. Toast sunflower seeds on a rimmed baking sheet in a single layer until golden, stirring occasionally, about 10 minutes. Set aside to cool. Meanwhile, stir mustard, lemon juice, salt, and maple syrup together in a small bowl; whisk in oil until emulsified.
Toss brussels sprouts, chard, and kale together. Stir in sunflower seeds and pour in dressing; toss to coat. Serve immediately.