To make the pickled red onions, fill a mason jar with the sliced onions. Add the sugar, salt, peppercorns, and vinegar to the jar. Add boiling water to the jar, cover, and shake. Refrigerate for at least 30 minutes; overnight for best flavor.
To roast the brussels sprouts, mix the sprouts in a bowl with the olive oil, salt, and pepper. Spread in an even layer on a parchment covered baking sheet and roast in a 425 degree oven for approximately 30 minutes, or until tender and well roasted, stirring/flipping over every 10 minutes or so.
Serve the pickled onions on top of the brussels sprouts. Use leftover onions for sandwiches or salads!