Melt the butter in a medium skillet over low heat. Once melted, stir in the vinegar and add the onions. Stir to evenly coat. Cover the onion with a lid and stir occasionally for about 1 1/2 to 2 hours. This step can totally be done the day before you plan on serving the sandwiches. Store the onions in an airtight container and just reheat in the microwave when ready to serve.

Preheat the oven to 375 degrees F. Line a baking sheet with tinfoil. Lay out the bacon strips in a row on the foil. Sprinkle each strip with a little bit of brown sugar and chili powder. Press the seasonings into the bacon.

Bake the bacon in the oven for 12-14 minutes until crispy.

Meanwhile toast your buns, slice tomatoes, and rip off leaves of lettuce. Add mayo to each top bun.

Let bacon cool on the sheet for a minute or two and then add it to the prepared buns along with the caramelized onion. Serve. Enjoy!