Whisk ingredients together in a bowl.

In a small saucepan, stir together the vinegar, water, salt, and your desired amount of sweetener. Cook over medium-high heat until the mixture reaches a simmer.

Place the thinly-sliced onions in a mason jar. Pour the hot vinegar mixture over the onions, screw on the lid, and shake the onions briefly until they are evenly coated with the vinegar mixture.

Let the onions marinate for 30 minutes. Serve immediately, or refrigerate the onions in a sealed container for up to two weeks.

Rinse steak in cold water and pat dry with paper towels. Marinate the steak in three-quarters of the marinade for one hour.

In a large skillet, heat olive oil on high heat until oil is smoking. Cook marinated steak for two minutes on each side. Remove from pan and allow to rest for 10 minutes.

Slice the steak very thinly. When ready to serve, build your lettuce cup by adding steak, veggies, herbs, and a quarter cup of the reserved marinade to lettuce and wrapping like a taco.