Tear spinach leaves into bite-sized pieces and place in large salad bowl (Amount of spinach can be varied based on how much dressing you like on your salad).
Add pears and onion.
In food processor with steel knife blade, combine sugar, vinegar, mustard, and salt.
With machine running, add oil through feed tube and process until dressing is thick (The slower you add the oil, the thicker the dressing).
Add poppy seeds.
Drizzle salad with dressing and toss well to coat.
Serve immediately.
You can make the dressing ahead and mix salad right before serving.