To make the sauce, combine all ingredients in a blender and process until smooth. The sauce can be made a day ahead and kept refrigerated in an air-tight container until serving time. The avocado in the sauce will not turn brown because of the acidity of the tomatillos and lime.
Prepare the grill for direct grilling with two temperature zones: one hot and one blazingly hot. A charcoal fire is preferred.
Brush the onion slices with olive oil and season with salt.
Squeeze the limes over the skirt steaks. Brush with olive oil and season with salt.
Grill the onion slices in the cooler part of the grill for a few minutes until they are nicely marked by the fire and softened.
Grill the elk skirt steaks over the hottest part of the fire. You may wish to grill them 2 or 3 at a time so that you can keep up. Grill the meat to medium-well, about 1 minute per side.
Remove the elk and onions from the grill. While the meat rests, coarsely chop the onions. After the meat has rested for a couple of minutes, cut each skirt into sections about 3 inches long, then very thinly slice the meat across the grain.
To assemble each taco, lay a little sauce into the tortilla. Add the elk meat and some grilled red onion. Top with queso fresco and fresh cilantro leaves. Enjoy!
Note: you can skewer a few tacos together to keep them from opening up or falling apart.
