Mix all dry seasonings in a bowl. Slice beef roast into 1"-wide strips and coat with spice mix. Lay them out, fully covering bottom of a slow cooker as much as possible. Place chopped jalapeno and sliced onion amongst strips. Fill slow cooker with hot water just enough to cover meat; add remaining dry spice mix. Cook on medium setting for 4 hours or until meat can be shredded. Shred meat in slow cooker. Set oven to 375F and use a fork to transfer meat to a foil-lined baking sheet. Discard excess liquid. Cook meat in the oven until top edges begin to crisp. In a large pan on the stove top, heat a tortilla on medium, placing some cheese in the middle. Once tortilla browns, remove from heat and add meat and toppings.