Preheat your grill or stovetop grill pan over medium-high heat. In a small bowl, mix together olive oil, chili powder, cumin, garlic powder, onion powder, salt, and pepper. Rub this mixture all over the flank steak.
Grill the steak for about 5-7 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from the grill and let it rest for 5 minutes before slicing it thinly against the grain.
While the steak is resting, heat a skillet over medium heat. Add the sliced bell peppers and onion, and sauté for about 5-7 minutes until they are tender and slightly caramelized. Season with salt and pepper.
In a blender or food processor, combine the avocado, cilantro, lime juice, Greek yogurt (or sour cream), garlic powder, and salt. Blend until smooth and creamy. If the dressing is too thick, add a little water to reach your desired consistency.
To assemble the salad, place the mixed greens in a large bowl or on individual plates. Top with the sliced steak, sautéed peppers and onions, and drizzle with the cilantro avocado dressing.
Serve immediately and enjoy your fresh and flavorful steak fajita salad!