In a large glass measuring cup or blender container, combine the oil, vinegar, celery seed, dry mustard, salt, honey, and onion. Blend with a countertop or immersion blender until emulsified and smooth, 1-2 minutes.
Pour into an airtight container and store in the refrigerator until ready to use, for up to one month.
For the coleslaw
In a large bowl, combine the cabbage and carrot. Add dressing to taste. It is best to start with a smaller amount of dressing, about ⅓, and stir well. Taste and add more if needed. Top with sunflower seeds and serve.
The coleslaw can be made two hours ahead of time. The thicker the cabbage is cut, the longer it will hold up in the refrigerator before breaking down.