Makes 6 servings.
First prepare a raspberry vinegar: combine red and white wine vinegars and raspberries. Cover and let sit 48 hours. Strain the vinegar, and store at room temperature. (You can also use commercial raspberry vinegar, which is available at specialty stores.)
To make Raspberry-Maple Dressing, whisk together in a bowl 1/2 cup of the raspberry vinegar, the oils, maple syrup, mustard, tarragon and salt.
Tear lettuce leaves by hand and toss in a large bowl with 3/4 cup of dressing.
To Serve: Divide lettuce evenly onto 6 chilled plates and top each with the blue cheese, onion rings and toasted pine nuts.
VARIATIONS: Roasted walnuts can be used in place of the toasted pine nuts.