Place spinach in a large salad bowl. In a small nonstick skillet, cook bacon over medium heat until crisp. Using a slotted spoon, remove to paper towels to drain. , In the drippings, saute onion until tender. Stir in the brown sugar, vinegar, salt, mustard, celery seed and pepper. Combine cornstarch and water until smooth; stir into skillet. Bring to a boil; cook and stir until thickened, 1-2 minutes. , Remove from the heat; pour over spinach and toss to coat. Sprinkle with bacon. Serve immediately.