In a shallow dish, combine 1/4 cup olive oil, 2 tablespoons lime juice, minced jalapeño, minced garlic, cumin, salt, and pepper.
Add the steak to the marinade, turning to coat. Cover and refrigerate for at least 30 minutes, or up to 4 hours.
Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the sliced onion and cook, stirring occasionally, until softened and caramelized, about 15-20 minutes. Remove from heat and set aside.
Preheat a grill or grill pan to medium-high heat.
Remove the steak from the marinade, discarding any excess marinade.
Grill the steak for 4-6 minutes per side for medium-rare, or to your desired doneness.
Remove the steak from the grill and let it rest for 5 minutes before thinly slicing against the grain.
While the steak rests, lightly toast the split rolls on the grill or in a toaster.
In a small bowl, combine mayonnaise, 1 tablespoon lime juice, and chopped cilantro.
Spread the lime mayonnaise on both sides of the toasted rolls.
Layer the sliced steak and caramelized onions on the rolls.
Serve immediately.
Notes
Adjust the amount of jalapeño to your spice preference.
For extra flavor, you can add a pinch of chili powder to the steak marinade.
Any leftover steak can be used in salads or tacos.