In a small bowl, whisk together the lime juice, minced garlic, cumin, chili powder, sea salt, and black pepper to create a marinade.
Rub the marinade evenly over both sides of the flank steak and let it marinate at room temperature for 10 minutes.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Sear the steak for 4 to 5 minutes per side until a deep crust forms, then transfer to a cutting board to rest for 5 minutes.
While the steak rests, mash the avocado in a small bowl with diced red onion and chopped cilantro until creamy.
Slice the rested steak thinly against the grain into bite-sized strips.
Wipe the skillet clean and place the sprouted grain tortilla in the pan over medium heat.
Sprinkle the shredded cheddar cheese over one half of the tortilla and top with the sliced steak strips.
Fold the tortilla in half and cook for 2 minutes per side until the exterior is golden brown and the cheese is fully melted.
Remove from the heat, slice into three wedges, and serve immediately with the fresh guacamole on the side.