Preheat a large nonstick saute pan, or preheat a grill or grill pan. Sear the flank steak on both sides for 3 to 4 minutes on each side or until medium-rare. Remove steak from heat and let rest for 10 minutes.

Meanwhile, combine vegetables, vinegar, olive oil, lime juice, cilantro, chili powder, salt, and pepper in a large bowl and mix well.

Cut steak against the grain into slices. Serve the steak and vegetable salsa with tortillas, chips and avocado, sour cream, and grated Cheddar, if desired.