Heat the oil in a skillet over medium-high heat.
Season the steak with ¼ teaspoon each salt and 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 ¼ teaspoon 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.
Warm the tortillas in the microwave on high for 1 ½ minutes. Slice the steak and serve on tortillas with the vegetables, sour cream, salsa, and guacamole.
