Dry off the chuck roast very well with paper towels on all side. Season with 1 teaspoon of the salt and 2 teaspoons of the fajita seasoning, rubbing all over.
Heat a large saute pan over medium-high heat for 3-4 minutes. Add avocado oil, followed by the chuck roast. Brown on all sides for 3-4 minutes until browned, turning down heat as needed. Transfer meat to a slow cooker.
Add onions and peppers to the same hot pan followed by remaining seasonings and salt and the black pepper.
Saute for 2-3 minutes to deglaze bottom of pan.
Add garlic and lime juice then stir. Transfer vegetables atop pot roast and cover slow cooker.
Cook on HIGH for 4.5 hours or LOW for 8 hours until roast is fork tender.
Transfer pot roast to a sheet pan and shred to your liking- discard any fat or gristle. Return shredded beef to slow cooker with cooked veggies and jus.
Serve fajita filling in warmed tortillas topped with cooked vegetables, queso fresco, fresh cilantro and limes.