Finely dice the bacon.

Render and crisp bacon in a large saute pan over medium heat.

Drain away bacon fat by tipping it out of the pan.

Don’t worry if you only get 80%-90% out.

Return pan with bacon in it to the heat, add the onions.

Cook, scraping until onions are glassy.

Add the vinegar, mustard, brown sugar and swirl to deglaze the pan.

Pull from heat and spill contents into a work bowl.

Whisk in the oil to taste.

Season with sea salt and ground black pepper, adjust acid-oil balance, and serve.