Whisk maple syrup, balsamic vinegar, mustard, and rosemary together to make marinade.
Place steak in zip-top freezer bag or large bowl and add marinade.
Cover and leave in fridge overnight.
Remove steak from marinade, reserving 1/4 cup of marinade, and cut into stir fry strips.
Heat coconut oil in wok or skillet over medium-high heat.
Stir fry steak strips in three or four separate batches, about two minutes apiece. Remove from skillet before putting in next batch.
Once all batches are done, return all the meat to the skillet with salt, pepper, and reserved marinade. Simmer and allow sauce to thicken for about 5 minutes.