For the flank steak/marinade:
1 1/2 pounds flank steak
1/3 cup neutral-flavored oil
Juice from 2 fresh limes
Juice from 1 fresh orange
2 cloves garlic, minced
1 teaspoon chili powder
1 teaspoon salt
For the avocado cream sauce:
2 avocados, halved, peeled and pitted
1 cup Greek yogurt or sour cream (or a combination of both)
2 cloves garlic, minced
Juice from 2 fresh limes
Salt and pepper, to taste
Remaining ingredients:
8 taco-sized corn tortillas, warmed
1 small white onion, finely chopped
Handful of fresh cilantro or parsley, finely chopped
Marinade the flank steak: Whisk together al steak marinade ingredients in a nonreactive bowl. Add steak to a large, resealable plastic bag and pour marinade into the bag. Close bag and massage marinade into steak. Place in refrigerator and chill for 1 hour.
While the steak is resting, make the avocado cream sauce. Add all sauce ingredients to a blender or food processor and pulse until smooth. Move sauce to a nonreactive container, cover and refrigerate until ready to use.
Once the steak has marinaded for an hour, remove from bag and shake off excess marinade. Preheat grill to medium-high. Grill for 12-15 minutes, flipping once in the middle of cooking. Remove from heat and let rest for 10 minutes before thinly slicing.
To assemble the tacos, place a few pieces of steak in a tortilla. Top with avocado sauce, onion and cilantro. Serve immediately.