Preheat the oven to 400 degrees . In a small saucepan, boil the pomegranate juice, sugar and lemon juice, stirring often, until reduced by about three-quarters (you'll have 1/4 cup glaze), about 8 minutes.
In a bowl, whisk the EVOO, onion and vinegar; season with salt and pepper. Trim off the woody stems from the sides of the Brussels sprouts stalk, if necessary. Place the stalk on a foil-lined baking sheet; brush the vinaigrette onto the sprouts.
Roast for 45 minutes or until the sprouts are tender, turning every 15 minutes. During the last 10 minutes of cooking, brush often with the glaze. Transfer to a platter; sprinkle with the pomegranate seeds. Serve with kitchen shears for removing the sprouts.