4cupssweet onionsuch as Vidalia or Walla Walla Sweets, divided use
1cupsliced leekwhite and light green parts only
fresh black pepper
Thinly slice all onions, reserving 2 cups of the sweet onions.
Heat a large skillet over low heat. Add olive oil and onions, except for the 2 cups reserved sweet onions. Sprinkle with salt. Cook slowly over low heat so that onions caramelize. This will take a long time. If the onions start to burn, add a little water to the pan to slow the cooking.
Deglaze pan with balsamic vinegar. Add reserved 2 cups of sweet onions, brown sugar, and thyme. Reduce over low heat until vinegar is absorbed/cooked off. Turn off heat and season to taste with salt and pepper. Pack in a jar with a tight-fitting lid, and store in fridge for up to two weeks.