In a large skillet, cook the diced bacon over medium heat until crispy. Remove the bacon with a slotted spoon and set aside on a paper towel-lined plate, leaving the bacon drippings in the skillet.

Add the chopped onion to the skillet with the bacon drippings. Cook over medium heat, stirring occasionally, until the onion is softened and translucent, about 3-4 minutes.

While the onions are cooking, whisk together the apple cider vinegar, sugar, salt, pepper, and water in a small bowl until the sugar and salt are dissolved.

Once the onions are softened, pour the vinegar mixture into the skillet. Bring to a simmer and cook for about 1 minute, stirring constantly, until the dressing slightly thickens.

Add the torn lettuce leaves to the skillet. Gently toss the lettuce with the dressing and onions until the leaves are just beginning to wilt. This should only take about 30-60 seconds. Be careful not to overcook the lettuce.

Remove the skillet from the heat immediately. Stir in the reserved crispy bacon.

Serve the wilted lettuce warm.