Heat a large skillet over medium high heat.
Sprinkle the chuck roast with 1/4 teaspoon salt and 1/4 teaspoon pepper.
Place chuck roast on the hot skillet and sear (approximately 30 seconds per side). Remove from heat.
Arrange the sliced yellow onion on the bottom of the crockpot.
Place the seared chuck roast on top of the onion.
Pour the green chiles over the chuck roast and sprinkle with oregano, 1/2 teaspoon salt, 1/2 teaspoon black pepper, and garlic powder.
Mix together water and corn starch. Pour over chuck roast.
Cook on low for 10-12 hours.
Turn of heat and shred beef with a fork or tongs. The meat with just fall apart and it will combine all ingredients.
Serve with corn tortillas, pic de gallo, sour cream, and cilantro.