Slice the flank steak into very thin strips against the grain and toss in a small bowl with the cumin, chili powder, sea salt, and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the thinly sliced red bell pepper and red onion to the skillet, sautéing for 3-4 minutes until they begin to soften and char slightly.
Push the vegetables to the side of the pan and add the steak strips in a single layer, searing for 2 minutes without moving to develop a crust.
Toss the steak and vegetables together and cook for another 1 minute, then remove the entire mixture from the skillet and set aside.
Wipe the skillet clean and return it to medium heat, then place the whole wheat tortilla flat in the pan.
Sprinkle half of the Monterey Jack cheese over one half of the tortilla, then layer on the steak and vegetable mixture.
Top the steak with the remaining cheese and fold the tortilla over to close the quesadilla.
Press down firmly with a spatula and cook for 2 minutes per side until the tortilla is golden brown and the cheese is fully melted.
While the quesadilla cooks, mash the avocado in a small bowl with lime juice and chopped cilantro to create a fresh guacamole.
Remove the quesadilla from the pan, let it rest for 1 minute, then slice into three wedges and serve with the guacamole on the side.