Place all marinade ingredients into a food processor. Process until mixture is thick and smooth. Add more lime juice or oil if mixture remains chunky.

Place flat iron steaks in a resealable plastic bag. Pour aji marinade over steaks and work it in making sure all surfaces of the meat are well coated. Seal bag and place into refrigerator for 6-24 hours.

Preheat grill for medium-high heat.

Remove steaks from bag and place onto grill. Cook 5-7 minutes per side or until steak reaches desired doneness. Place tortillas in a large piece of aluminum foil, wrap and place on the cooler part of the grill for about 5-7 minutes, turning once.

Once cooked, remove steaks from grill and place onto a cutting board. Remove tortillas and set aside. Slice steaks into thin strips and serve in warmed tortillas with guacamole. If preferred, make a double batch of the aji sauce and reserve half to serve with the tacos.