Add the store-bought bread and butter pickles and the rough chopped mixed garden vegetables to a food processor.
Pulse 1 to 2 times until you get a small chop. Then add the caster sugar and ground cinnamon.
Pulse again 1 to 2 pulses until you end up with a lightly coarse mixture. Scrape the blended pickle vegetable mixture into a medium sized mixing bowl (not metal). Add the pickled mustard seeds, pickle juice, mustard, vinegar, and onion powder. Season mildly with freshly cracked black pepper to taste.
Using a rubber spatula, stir the relish well. Cover and let the relish marinate at room temperature for two hours. Then check the seasoning again and add extra cinnamon, sugar, onion powder or pepper if necessary. Transfer the relish to a jar or small bowl with a lid and refrigerate overnight. The flavors marry best the longer the relish sits.
Store refrigerated for up to two weeks.
This relish accompanies all kinds of grilled vegetables and meats and tastes especially impressive with a plank finished pork tenderloin if you enjoy cooking with cedar. The flavors and aromas open up with a bright and well chilled Grenache or a spiced Old Vine Zinfandel.