Place the flank steak in a zip lock bag then add the garlic, cumin, scallion, olive oil, paprika, salt and pepper. Marinade for at least 1 hour or overnight.
Remove steak from marinade bag and remove as much of the marinade as possible. Discard remaining marinade. Place the steak on the preheated grill and cook 5 minutes on each side for medium doneness or cook until desired degree of doneness.
Remove from grill and allow the steak to rest for 10 minutes. Slice the steak against the grain and set aside.
Avocado Sauce
Place all the ingredients for the sauce in a food processor and process until smooth. Transfer to a serving bowl.
Combine the rest of ingredients in a bowl to make a tomatao salsa.
To Serve: Let everyone assemble their own tacos with steak, avocado sauce and tomato salsa