Dressing - Place cranberry juice and dried cranberries in a small pot and bring to a boil. Remove from heat and allow it to cool. Stir in vinegar and Dijon mustard. Gradually whisk in canola oil so that the mixture becomes a dressing. Season with salt and pepper, to taste.
Salad - Rinse spinach and frisée; spin dry. Remove thick stems and cut larger leaves into bite-size pieces. Add avocado and onion slices. Gently toss salad ingredients with the dressing and serve.
Tip: If preparing in advance, sprinkle avocado slices with lemon juice to prevent discoloration. Mix the salad ingredients with the dressing just before serving to keep the leaves fresh and crisp.