Place the chuck roast in a large pot. Cover it with water and bring it to a boil over high heat. Immediately reduce the heat to medium and let it simmer for 2.5-3.5 hours or until it falls apart. Remove it from the heat.

Remove the chuck roast from the pot and set it aside to cool for 10-15 minutes. Carefully use two forks to shred the meat. Remove any large chunks of fat. Set the shredded beef aside until completely cooled.

Place the guajillo and arbol chiles in a medium heat-proof bowl. Add the hot water and let them soak for 10-15 minutes or until softened.

Drain the chiles using a strainer and reserve their liquid. Place the rehydrated chiles in the blender. Set the liquid aside.

Add the olive oil, sesame seeds, salted peanuts, and garlic powder to the blender. Process until you get a smooth mixture. Season with salt to taste. Set it aside.

Toss the shredded beef and white vinegar in a large bowl until well combined. Add the tomatoes, white onion, cilantro, and serrano. Mix well. Season with salt and pepper to taste.

Place the beef-vinegar mixture in the fridge for at least 30 minutes.

Remove the salpicon from the fridge and place it in a large serving bowl. Separate the lettuce leaves and place them on a separate plate. Let everyone assemble their own tacos using the lettuce as "tortillas." Drizzle with sauce to taste and top with sliced avocado.