Combine the sugar, salt, mustard, lemon peel, onion, and lemon juice in a blender. Cover and process a few seconds to combine the ingredients. With the blender running on a low speed, slowly pour the oil through the feed hole in the lid in a steady stream. Continue to blend until the ingredients are incorporated and the dressing is thick. Transfer the dressing to a jar. Stir in the poppy seeds. Use immediately or cover and store in the refrigerator for up to 10 days.