Red Pepper Relish

1 lb red peppers, finely chopped
1 onion, finely chopped
1 cup apple cider vinegar
1/4 cup white sugar
1/4 cup brown sugar
2 teaspoons mustard seeds
1 teaspoon kosher salt
1/2 teaspoon red pepper flakes, crushed
pinch cayenne or chipotle powder

Combine all ingredients in heavy saucepan. Simmer 45 minutes or until juice is syrupy and liquid has almost evaporated. (Turn the exhaust fan on�smells quite vinegar-y at first!)