Whisk together first 6 ingredients; gradually whisk in 1/2 cup oil until blended. Set aside.
Rinse Brussels sprouts thoroughly, and remove any discolored leaves. Trim stem ends; cut in half lengthwise. Combine Brussels sprouts and next 3 ingredients in a large bowl; toss to coat. Transfer Brussels sprouts to 1 or 2 large rimmed baking sheets, spreading into 1 layer.
Roast at 450° for 25 to 30 minutes or until tender and browned, stirring once. Transfer roasted Brussels sprouts to a serving bowl, and cool slightly. Add tomatoes, green onions, and parsley; toss to blend. Add desired amount of dressing, and toss to coat. Serve warm or at room temperature.