To make the pickled red onions, fill a mason jar with the sliced onions. Add the sugar (1 tablespoon), salt (1 tablespoon), peppercorns (1/2 teaspoon), and vinegar (1 cup) 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 (2 tablespoons), 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!
To save on time, make the pickled onions beforehand. They will last for months in the fridge.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.