To make the pickled onions, combine the vinegar, sugar and 1 teaspoon salt in a small saucepan over medium-high heat. Bring to a simmer and cook until the sugar dissolves, about 2 minutes. Place the onions in a 16-ounce jar or heatproof container. Pour the pickling liquid over the onions and let sit until onions are softened and juices have cooled to room temperature, about 20 to 30 minutes. (The onions can be refrigerated for up to a week.)
Whisk the miso, sriracha, soy sauce honey and lemon juice together in a large bowl. Add the tuna and fold until completely coated. Set aside in the fridge while preparing the rest of the ingredients.
To make the fried wontons, heat the oil over medium heat in a medium skillet until the oil is shimmering and hot, about 5 to 8 minutes. Working in batches, fry the wonton strips, stirring occasionally, until wontons are light golden brown and crisp, about 1 minute. Use a slotted spoon to remove and drain on a towel-lined plate. Season with salt.
Divide the zoodles into two bowls and season with salt. Divide the scallions, carrots, fried wontons, pickled onions and avocado around the edge of the bowls. Spoon the tuna into the center and top with a pinch of white and black sesame seeds.