Heat the oil in a large skillet over medium-high heat. Season the steak with 1/4 teaspoon of the salt and the pepper. Cook for 3 minutes. Turn and move to the sides of the skillet. Add the onion and cook, stirring occasionally, for 2 minutes. Add the bell pepper, season with the remaining salt, and cook until the vegetables are softened and the steak is medium-rare, 3 to 4 minutes more. Transfer the onion and bell pepper to a bowl. Transfer the steak to a cutting board.
Place the tortillas in a microwave and warm on high for 1 1/2 minutes. Slice the steak and serve with the onion, bell pepper, tortillas, sour cream, salsa, and guacamole.