Thinly slice the flank steak against the grain into bite-sized strips.
In a small bowl, toss the steak with sea salt, black pepper, cumin, chili powder, and lime juice until evenly coated.
Heat half of the olive oil in a non-stick skillet over medium-high heat; add the sliced red bell peppers and onions, sautéing until tender-crisp, then remove from the pan.
Add the remaining oil to the same skillet and sear the steak strips for 3-4 minutes until browned and cooked through.
Wipe the skillet clean and place the whole wheat tortilla in the pan over medium heat.
Sprinkle the Monterey Jack cheese over one half of the tortilla, then top with the cooked steak and sautéed vegetables.
Fold the tortilla in half and cook for 2 minutes per side until the cheese is melted and the exterior is golden and toasted.
While the quesadilla cooks, mash the avocado in a small bowl with a pinch of salt and the fresh cilantro.
Slice the quesadilla into wedges and serve immediately with the fresh guacamole on the side.