Heat the olive oil in a large skillet over medium heat.
Add the diced onions to the skillet and cook for 8-10 minutes, stirring occasionally, until the onions are golden brown and caramelized.
Add the brown sugar, balsamic vinegar, Dijon mustard, Worcestershire sauce, black pepper, garlic powder, red pepper flakes, cumin and oregano to the skillet and stir to combine.
Reduce the heat to low and cook for 10-15 minutes, stirring occasionally, until the mixture has thickened and the onions are cooked through.
Remove the skillet from the heat and let cool to room temperature before serving.
Interesting Facts
This relish can be stored in an airtight container in the refrigerator for up to 5 days.
The sweetness of the onion relish can be adjusted by adding more or less brown sugar.
This relish pairs well with grilled meats, vegetables, and even eggs.