Orange Walnut Salad with Sweet-and-Sour Dressing

3 small heads bibb lettuce, cleaned and torn into bite-size pieces
1 1/2 pounds fresh spinach, cleaned and torn into bite-size pieces
3 oranges, peeled, sectioned, and seeded
3/4 medium red onion, sliced and separated into rings
3/4 cup walnuts, coarsely chopped
3 teaspoons butter

Dressing:
1 1/2 cups vegetable oil
3/4 cup vinegar
3/4 cup sugar
1 1/2 teaspoons salt
1 1/2 teaspoons celery seed
1 1/2 teaspoons dry mustard
1 1/2 teaspoons paprika
1 1/2 teaspoons grated onion

In a large bowl, combine the lettuce, spinach, oranges, and onion. Using a saucepan, saute the walnuts in butter until lightly browned and add to the lettuce mixture. To make the dressing, combine all of the ingredients in a jar, shake well and chill in refrigerator. Shake before serving. Pour on top of salad and serve.