Slice the flank steak into very thin strips against the grain and place in a bowl.
Toss the steak strips with the ground cumin, chili powder, garlic powder, sea salt, and black pepper until evenly coated.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the sliced bell peppers and red onions to the skillet, sautéing for 3-4 minutes until softened and slightly charred.
Remove the vegetables from the skillet and set aside; in the same pan, sear the steak strips for 2-3 minutes until browned and cooked through.
In a small bowl, mash the avocado with the lime juice and chopped fresh cilantro to create a simple guacamole.
Wipe the skillet clean and place the whole wheat tortilla in the pan over medium heat.
Spread the Monterey Jack cheese over one half of the tortilla, then layer on the cooked steak, peppers, and onions.
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 triangles, and serve immediately with the fresh guacamole on the side.