In a small bowl whisk the marinade ingredients. Place the steak pieces in a large resealable plastic bag and pour in the marinade. Press the air out of the bag and seal tightly. Turn the bag to distribute the marinade, place in a bowl, and refrigerate for 2 to 8 hours. Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over high heat (450° to 550°F).
In a food processor combine the cream ingredients and process until smooth. Transfer to a bowl and cover with plastic wrap until ready to use.
Remove the steaks from the bag and discard the marinade. Brush the cooking grates clean. Grill the steaks over direct high heat, with the lid closed, until cooked to your desired doneness, 4 to 6 minutes for medium rare, turning once or twice. Remove from the grill and let rest for 3 to 5 minutes. Cut the steaks across the grain into thin slices.
Spread half of each tortilla with avocado cream and top with steak slices, onion, and cilantro. Serve immediately.