On our raw-meat cutting board (that will be sanitized afterwards) we thinly slice the flank steak, against the grain, into ¼ inch by 2 inch strips. We place these in a bowl and set aside for a few minutes while we wash up and make our sauce.
In a measuring cup, we combine the soy sauce, sherry, sugar, sesame oil, and some pepper. We pour this over our meat, stir with a spoon, and refrigerate, covered with plastic wrap, while we prep the remaining ingredients.
We mince the ginger and put it in a small prep bowl. Next, we wipe the mushrooms clean with a damp cloth (they are not that dirty) and then slice them thinly on our cutting board. To save time, we use our food processor to cut the cabbage and carrots. We cut the cabbage into quarters, remove the core, and cut each piece into smaller wedges that will fit into the feed-tube of our food processor. Using the slicing attachment to our processor, we slice the cabbage, piece by piece, through the feed-tube. We rinse and pat dry the packaged carrots and then, using the shredding attachment, we shred the baby carrots a few at a time, through the feed-tube. Lastly, we quarter the light parts of the scallions and then slice diagonally all the way up to incorporate some of the darker green parts.
When all the ingredients are prepped, we remove the meat, which is still in the marinade, from the fridge and stir. All the meat is defrosted at this point.
We heat half the oil over medium-high heat in a large pot. We pick out the meat, leaving behind the marinade, and place them on a paper-towel lined plate. We pat dry as best we can, and add salt and pepper. We add the meat to the hot oil.
We should have done this in batches, because the meat did not brown the way we would have liked. We stir-fry for just a minute or two and then remove it to a clean plate. We dump out the liquid from the pot and pour over the nearly cooked meat. We put the pan back on the heat and add the rest of the oil.
When hot, we add the ginger, mushrooms, cabbage, and carrots. We stir-fry for a few minutes and then add the soy-sherry sauce that had the raw meat in it. We lower the heat to medium and cook, stirring often for a few minutes until the vegetables are tender. Finally, we add the meat with the juices and the scallions. We toss to heat through.
We heat the tortillas in the microwave on a dinner plate between dampened paper towels for a minute. We spread hoisin sauce on a tortilla, top with mu shu, and roll up.