In a small bowl, whisk together the olive oil, lime juice, garlic powder, cumin, sea salt, and black pepper to create a marinade.
Coat the flank steak thoroughly with the marinade and let it sit for at least 15 minutes at room temperature.
Preheat a grill or cast-iron skillet over medium-high heat and sear the steak for 4-5 minutes per side until it reaches your desired level of doneness.
While the steak rests for 5 minutes, mash the avocado, Greek yogurt, and cilantro in a small bowl until the crema is silky and smooth.
Slice the bolillo roll in half lengthwise and toast the cut sides on the grill or in a pan until golden brown.
Thinly slice the rested flank steak against the grain into bite-sized strips.
Spread the avocado crema generously on both sides of the toasted roll, then layer with the steak, romaine lettuce, tomato slices, and red onion.