Everyone's Favorite Spinach Salad

1 pound spinach
1 can mandarin oranges
bacon bits

Dressing:
1 small onion, chopped
2/3 cup sugar
1 tsp. salt
1/4 tsp. pepper
1/2 tsp. celery seed
1/3 cup white vinegar
2/3 cup canola oil
2 teaspoons mustard