Marinate the steak in the lime juice, salt, garlic, and chili powder in a sealed plastic bag for 20 to 30 minutes.
Meanwhile, heat a large cast-iron skillet over high heat for 5 minutes. Add 2 teaspoons of the vegetable oil to the pan. Add the red onion and bell peppers; cook, tossing frequently, for 5 minutes. Add the corn and continue to cook until the peppers are charred and soft, about 3 more minutes. Transfer the vegetables to a bowl and keep warm.
Wipe the skillet with a paper towel and heat for another minute. Add the remaining teaspoon of vegetable oil. Remove the steak from the marinade and pat dry with paper towels. Lay the steak in the pan, reduce heat to medium-high, and cook about 8 minutes for medium-rare, turning once halfway through. Transfer to a board and let rest for 5 minutes.
Slice the steak across the grain and arrange on a platter with the peppers and lime wedges. Make tacos with the warm tortillas, avocado, Monterey Jack, salsa, jalapenos and, if desired, sour cream.