Preheat your grill or grill pan over medium-high heat. In a small bowl, mix together olive oil, salt, black pepper, garlic powder, and onion powder. Rub the mixture all over the flank steak.
Grill the steak for about 5-7 minutes on each side for medium-rare, or until desired doneness. Remove from the grill and let it rest for 5 minutes before slicing thinly against the grain.
In a blender or food processor, combine cilantro, parsley, red wine vinegar, red pepper flakes, avocado, olive oil, minced garlic, and lime juice. Blend until smooth. Adjust seasoning with salt and pepper to taste.
Heat a large skillet over medium heat. Place one tortilla in the skillet and sprinkle half of the shredded cheese evenly over it. Layer half of the sliced steak on top of the cheese, then drizzle with a few tablespoons of avocado chimichurri. Top with another tortilla.
Cook for about 3-4 minutes, or until the bottom tortilla is golden brown and the cheese is melted. Carefully flip the quesadilla and cook for an additional 3-4 minutes on the other side. Remove from the skillet and repeat with the remaining ingredients.
Cut the quesadillas into wedges and serve warm with extra avocado chimichurri on the side.