Bring approximately 5 cups of water to a boil. Meanwhile, peel and thinly slice the red onion.
Mix the rice vinegar with the sugar in a medium jar or bowl. Set aside.
Place the sliced onions in a fine mesh strainer and slowly pour the boiled water over the onions. Allow the water to fully drain before transferring them to the container with the vinegar. Stir gently and allow onions to rest for at least 30 minutes to overnight. Keep stored in the refrigerator until ready to use.
For the vinaigrette
Whisk together all ingredients for the vinaigrette in a medium bowl until fully combined. Season with additional salt, pepper, and red wine vinegar, to taste.
For the Salad
Add the cucumbers to a large bowl and drizzle with 1-2 tablespoons of the prepared vinaigrette. Toss to combine. Transfer to a large salad bowl and top with the sliced avocado, pickled red onions, and additional vinaigrette. Gently toss to combine and top with crumbled Casero cheese. Season with additional salt and pepper, to taste. Enjoy!