Peel your red onions and slice them using a mandoline slicer. You can slice the onions into whole rounds (for larger jars) or cut them in half and slice them into half moons (smaller jars).

Divide the sliced onions between jars with a tight lid. Divide the mixed peppercorns and mustard seeds between the jars.

In a small saucepan, combine white vinegar, apple cider vinegar, maple syrup, and sea salt. Heat gently over medium heat, stirring until salt dissolves. The mixture does not need to boil.

Carefully pour the warm brine over the onions until fully covered. Using a skewer or a chopstick, gently push the onions so the spices mix through the jars with the onions. You do not want them just sitting on the top.

Let the jars cool, then refrigerate for 1 hour or overnight.

Serve as desired.