Preheat oven to 300°F.
Cut the chuck roast into four large pieces. In a small bowl, combine two teaspoons salt, chili powder, cumin, oregano, garlic powder, and onion powder. Rub the spice mixture evenly over the chuck roast.
Heat oil in a large Dutch oven over medium-high heat. Add the roast and sear until deeply browned, about 3–4 minutes per side.
Carefully pour in the beer, scraping up any browned bits from the bottom of the pot. Simmer until reduced by half. Stir in enough beef stock to go about halfway up the beef. Add in the onion, garlic, canned chiles and remaining salt. Give it a stir.
Bring the liquid to a simmer, cover, and transfer to the oven.
Cook for 2 to 2 1/2 hours or until fork tender.
Skim excess fat from the cooking liquid if needed. Shred the beef with two forks directly in the pot. Stir to combine with the braising liquid. Season to taste with salt and pepper.
In a small bowl, mash the avocados with a few tablespoons of lime juice and a pinch of salt.
Warm the tortillas in a dry skillet or char them over a gas flame. Spread each tortilla with smashed avocado. Top with shredded beef. Spoon tomatillo salsa on top ad sprinkle with cheese.