Preheat your grill or a skillet over medium-high heat. Rub the flank steak with olive oil, and season generously with salt and pepper.
Grill or sear the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from heat and let it rest for 5-10 minutes before slicing.
In a large skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, for about 15-20 minutes until they are soft and caramelized. Stir in the brown sugar and cook for an additional 2-3 minutes. Remove from heat.
In a large bowl, combine the mixed salad greens, cherry tomatoes, and caramelized onions. Toss gently to mix.
Slice the rested steak against the grain into thin strips and add it to the salad. Top with crumbled blue cheese and drizzle with balsamic vinaigrette before serving.