Thinly slice the top sirloin steak into bite-sized strips against the grain for maximum tenderness.
In a small mixing bowl, whisk together the chili powder, ground cumin, sea salt, black pepper, and lime juice.
Toss the steak strips in the spice mixture until evenly coated and let marinate for 5 minutes.
Heat the avocado oil in a large non-stick skillet over medium-high heat.
Add the sliced red bell pepper and yellow onion to the skillet, sautéing for 3-4 minutes until softened and slightly charred.
Remove the vegetables from the pan and set aside, then add the steak strips in a single layer.
Sear the steak for 2 minutes per side until browned and cooked through, then remove from the pan.
Wipe the skillet clean and place the whole wheat tortilla in the pan over medium heat.
Sprinkle the Monterey Jack cheese over half of the tortilla, then top with the cooked steak and sautéed vegetables.
Fold the tortilla in half and press down firmly with a spatula, cooking for 2 minutes per side until the exterior is golden and crispy and the cheese is melted.
Slice the quesadilla into three wedges and serve immediately with a dollop of plain Greek yogurt.