Pat the flank steak dry with a paper towel and rub both sides evenly with the ground cumin, chili powder, garlic powder, sea salt, and black pepper.
Heat one teaspoon of olive oil in a large cast-iron skillet or grill pan over medium-high heat until shimmering.
Add the sliced bell peppers and red onions to the skillet, sautéing for 5-7 minutes until the vegetables are tender and develop dark, charred edges.
Transfer the cooked vegetables to a plate and add the remaining teaspoon of olive oil to the same skillet.
Place the seasoned steak in the skillet and sear for 4-5 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the heat and let it rest on a cutting board for at least 5 minutes to lock in the juices before slicing thinly against the grain.
While the steak rests, warm the corn tortilla in a separate dry pan over medium heat for about 30 seconds on each side until pliable.
Arrange the sliced steak and charred vegetables on a platter, then top with fresh avocado slices, a dollop of Greek yogurt, and a bright squeeze of lime juice.