Place 1/2 cup of filling on the rice paper, slightly below the center, working from the edge closest to you. (Where the smile on a smiley face goes.) Top the mound with 2 avocado slices.
Pick up the edge closest to you and fold the rice paper up and over the mound. Fold the right and left sides toward the center. Pull the spring roll gently toward you as you roll it up burrito-style. Protect each spring roll from sticking to the others with a layer of moist parchment. Repeat the procedure until all the avocado slices and filling have been used. Cover the spring rolls with plastic wrap and refrigerate. The spring rolls can be made up to 2 hours before serving.
Just before serving, cut each spring roll in half at an angle. Serve with additional dressing on the side.
For the Charred Red Onions:
Turn on an exhaust fan if you have one. Preheat a large cast-iron skillet or grill pan until blazing hot.
Toss the onions in a bowl with the olive oil, salt and pepper. Add the onions to the hot skillet and stir frequently. Cook the onions until they are black and charred. Transfer the onions to a small bowl and set them aside to cool. Makes about 1 cup charred onions.
For the Thousand Island Dressing:
Mix the first 4 ingredients in a medium bowl and season to taste with salt and pepper. Use immediately, or transfer to a container with a tight-fitting lid and refrigerate for up to 2 weeks. Makes about 2 1/4 cups dressing.