In a medium-sized bowl, combine the soy sauce, sugar, lime juice, and chopped serrano peppers. Whisk until the sugar is dissolved
Mix in the beef strips and set aside
Heat the oil in a large skillet until hot, about a minute. Add the beef strips and cook until no pink is showing about 3-4 minutes
Remove the beef and set aside, leaving the juices in the skillet, and cook them until reduced and thick. Add the sliced scallions and minced garlic and cook for one minute until fragrant.
Add the mushrooms and cook for 3 more minutes. Mix in the rest of the veggies and cook them until tender but still crisp, about 2 more minutes
Transfer the beef back into the skillet and cook a couple of minutes more until the meat is heated through. Season with salt and pepper