In a large saute pan, cook the pancetta on medium heat, stirring occasionally, until it starts to brown, this should take about 15 min. Keep the rendered fat in the pan.
Add the onions, cook for 5 minutes and then add the mustard.
Add both vinegars to the mix (The vinegar aroma will be very strong) reduce the vinegars for 8 minutes or until the liquid is reduced by 1/2.
Reduce the heat to low, add the brown sugar to the pan and stir until all the sugar is dissolved.