Place flank steak in an airtight container and marinate in refrigerator with fajita marinade overnight.
Remove steak from marinade and grill over medium-high heat for 4-5 minutes per side or until it reaches desired temperature. Allow to rest for 5-10 minutes before slicing against the grain.
Heat olive oil in a skillet over medium-high heat. Add peppers and onion and sauté until tender.
Serve grilled steak over rice and veggies. Top with guacamole and salsa, if desired.