Apple, Cheese and Toasted Walnut Salad

1/4 cup sugar

3 tablespoons white wine vinegar

1 tablespoon finely chopped red onion

1/2 teaspoon salt

1/2 teaspoon ground mustard

1/4 to 1/2 teaspoon celery seed

1 medium head romaine lettuce, torn into bite-size pieces (about 10 cups)

4 cups mixed salad greens (from 10-oz bag)

1 container (4 oz) blue or Gorgonzola cheese crumbles

1 cup walnut halves, toasted

2 red apples, cut into bite-size pieces