You can substitute different vinegars in this recipe, but don't substitute with lemon juice because the flavor will be too strong. Use a neutral vegetable oil, not olive oil. Olive oil will make the dressing too heavy and will overpower the dressing.
In bowl of food processor, combine vinegar, honey, mustard, salt, pepper and onions. Whizz in processor briefly to mix.
With motor running, slowly add the oil in a steady stream until it is all incorporated. You should add is slowly enough that as it is added to the mixture in the food processor is it mixed in right away: you shouldn't be able to see the oil as a separate ingredient. (Don't worry if you add it too quickly - the dressing will still taste fine: it will just separate more easily.)
Pour the dressing into a bowl or jar and stir in the poppy seeds. Store unused dressing in the refrigerator for up to one week. If the dressing separates, shake it gently to mix.
In a small mixing bowl, whisk together vinegar, honey, mustard, salt, pepper and onions until combined.
While whisking, slowly add the oil in a steady stream until it is all incorporated.
Stir in the poppyseeds.