In a medium pan over medium-high heat, saute onion in oil until softened and slightly carmelized, about 10 minutes.
Season with salt and pepper.
While onions are cooking, combine vinegar, mustard, mustard powder, salt, pepper, garlic, cranberry juice, and honey.
Toast pumpkin seeds lightly.
Add vinaigrette and dried cranberries to onions and heat through.
Place spinach in a salad bowl (note: I usually can only find 7oz packages of spinach), pour warmed vinaigrette mixture over salad, add pumpkin seeds, and toss lightly. Serve immediately.
Notes: this makes more than 4 servings, but the original recipe said 4. Once I didn't have any cranberry juice, so I used jellied cranberry, and it was fine - the dressing was a little thicker, but still very good!