After thinly slicing the onions, put them in one or more mason jars or glass bowls. Add the garlic and mustard seeds or peppercorns if using them as well (plus any other spices, see post for ideas). Set aside.
Add the water, vinegars, sweetener, and salt to a saucepan. Boil until the sweetener and salt has dissolved. Remove from heat and allow to cool to room temperature.
Once the brine has reached room temperature, pour it over the onions until they are just covered (you can press them down as necessary to achieve this). Put the lid(s) on if using mason jar(s) or cover the bowl(s) with lid(s) or plastic wrap.
Refrigerate for 8 hours or overnight. The brine will turn a pinkish color.