Chop the romaine lettuce into 2-inch-wide pieces and place in a large serving bowl. Set aside.
Slice the steak across the grain into ¼-inch strips and then cut the strips in half so they are 4 inches long or less. Set aside.
Heat the avocado oil in a large skillet over medium heat. Add the onions and saute for 2 minutes, until they start to turn translucent.
Increase the heat to medium-high and add the beef, sea salt, pepper, and coconut aminos. Stir-fry the beef until it starts to brown but is still red in the center, then add the bell peppers and green beans.
Cook for 1 to 2 more minutes until the green beans turn a bright color and the beef is cooked to your liking.
Using tongs or a slotted spoon, transfer the contents of the stir-fry to the bowl of lettuce.
Drizzle with olive oil and balsamic vinegar and toss all the
ingredients together.
Serve on individual plates. Leftovers can be refrigerated
for up to four days.