Season the skirt steak with 1 teaspoon salt, chili powder and cumin.
Heat half the oil on a griddle over medium-high heat and cook the bell pepper and onion with the remaining salt for 7 to 9 minutes until soft and browned.
Move the vegetables aside, add the remaining oil and sear the steak for 2 to 3 minutes per side until it reaches 145°F.
Rest the steak for at least 3 minutes, slice it thinly across the grain and toss the slices with the lime juice.
Mash one quarter of the avocado onto the bottom half of each bolillo roll.
Divide the steak, peppers and onions among the rolls, close them and toast on the griddle for 1 minute per side.