Preheat oven to 400. Toss brussels sprouts, onions, salt, and 2 teaspoons oil in a medium bowl. Season with pepper. Arrange vegetables in a single layer on a rimmed baking sheet. Roast, stirring every 10 minutes, until golden brown and tender, 30 to 35 minutes.
Transfer roasted vegetables to a serving dish, and toss with herbs, vinegar, warm chicken stock, sunflower seeds, and remaining 1 teaspoon oil. Serve immediately.