Heat a medium sized sauce pot over medium-high heat. Add black peppercorns and coriander seeds. Toast for 2 - 3 minutes until fragrant and lightly smoking. Add spices to a pint sized mason jar, and return pot to heat.
Add vinegars, water, salt, (and sugar if using), and bring to a boil.
While heating the brine, peel, halve and thinly slice the red onion. Crush and peel two cloves of garlic. Place onions and garlic into the mason jar with the spices.
After the brine comes to a boil, immediately turn off the heat and pour over the onions and spices. Allow to cool for several hours. Cover and refrigerate.
Notes
These will keep for months stored in an airtight container in the fridge.