Considering how expensive poppy seed dressing can be at the store, it is amazingly simple to make at home. I like mine a little sweeter than the stuff they sell at the store. If you want it more tart, cut back a bit on the sugar. Don't skip the dry mustard...that is the ingredient that acts as the emulsifier that lets the dressing not separate. If you don't have any dry mustard, you can either grind up some mustard seeds in a spice grinder, or use about a teaspoon of Dijon style mustard.
In a blender, mix the onion, mustard, salt, vinegar and sugar until it is combined. While continuing to run the blender, drizzle in the oil in a slow steady stream until it is all incorporated. Kill the blender. Stir in the poppy seeds.