Gently toss the first six ingredients in a bowl with just enough dressing to coat. On its own, this mixture can be served as a hamburger salad but the spring rolls make it special.
Assemble the spring rolls: Select a dish or pan that is large enough for the rice paper to lie flat inside. Inspect each rice paper carefully and discard any sheets that have holes or cuts. Add about 1 cup of very warm water to the dish or pan and soak 1 sheet of rice paper for about 20 seconds, or until the paper has softened but has not become completely gummy bear soft. Place the rice paper on a cutting board and rub it gently with your wet hands to flatten it and work in more water.
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.