If using, put the peppercorns in the bottom of the mason jar.
Slice the onion and shallot (optional) and pepper (optional) thin and place in the jar.
In a small saucepan add the water (120g), vinegar(s) (120g), grenadine (40g) and salt (10g). (Optional: add red pepper flakes.) Bring to a boil to a boil. Remove from heat and let cool slightly. (If you'd like, add a teaspoon of red pepper flakes.)
Pour brine over the onions in the jar and seal.
Store in the 'fridge for up to 6 weeks.