Finely chop 1 onion.
In a small bowl mix together 1 tsp salt, 1 tsp chili powder, 1 tsp cumin and 1⁄2 tsp black pepper and rub all over 2# chuck roast.
Heat 1 Tbsp olive oil in a large sauté pan over med-high heat. Sear seasoned chuck roast on all sides (~2mins per side).
To slow cooker add the following: browned beef, 1 diced onion, 4 oz can diced green chiles, 10 oz can diced tomatoes w/ chiles.
Cook on low 8-10 hours.
Chop 1 bunch of cilantro & 1 large tomato. Quarter 2 limes.
Shred beef.
Warm 8 corn tortillas on griddle or warm pan.
Serve shredded beef in corn tortilla topped with guacamole, fresh cilantro and chopped tomato. Squeeze fresh lime juice on top. Roll into a burrito or eat like a taco.