Combine vinegar and water in a saucepan. Stir in sugar and salt; add mustard seeds, dill, peppercorns and habanero pepper flakes. Bring to the boil; stir until sugar and salt are dissolved. Remove from heat; cool to room temperature, about 15 minutes.
Divide habanero peppers and garlic clove between 2 sterilised jars. Divide avocado slices evenly between jars.
Stir vinegar mixture to evenly disburse seasoning; pour into jars. Close lids and place in the fridge. Chill until flavours combine, about 24 hours.
For more of a kick, pierce each habanero chilli pepper with a fork a few times.
The brine can be reused by simply adding more avocados and allowing them to rest for 24 hours before consuming. These will last about 5 days in the fridge.