1. Peel, quarter and thinly slice the onions approximately ⅛" - ¼" thick. (Or dice the onions if you prefer)
2. Heat a large or sauté pan on low to medium-low heat
3. Add the onions and sweat (soften without cooking) the onions (without oil) until dry, making sure not to let them caramelize or burn - keep heat on medium-low (or low if you need to adjust the temperature). This will take about 10 minutes
4. Stir in vinegar, mustard seeds, brown sugar and salt
5. Continue to cook over medium-low heat, stirring occasionally, until liquid has evaporated and mixture starts to reach a jam-like consistency, 20-25 minutes
6. Set aside and let cool completely before use

Note: If not using the same day, store in an air-tight container and chill. It will keep for up to 3 weeks refrigerated