Place the steak in a 9×13-inch baking dish or other shallow pan. Whisk together the lime juice, olive oil, garlic, chili powder, and salt. Pour this over the steak and turn a few times so it’s evenly coated. Cover and refrigerate for at least 1 hour or up to 24 hours.
When ready to cook, heat the broiler to high and place a rack a few inches below. Remove the steak from the fridge and set it on the counter to warm while you prepare the vegetables.
Cut the peppers in half, and remove the stems and seeds. Peel and cut the onion into rough wedges. Toss the vegetables with olive oil and sprinkle with salt. Lay the vegetables in a single layer on a baking sheet (with the outsides of the bell peppers facing up, like upside-down cups). Broil the vegetables until the tops are charred, 10 to 12 minutes. Remove vegetables as they are done and rearrange the remaining vegetables as needed to char directly beneath the heat. Cool slightly, then cut the vegetables into thin slices and arrange on a serving platter.