If you don't have buttermilk in hand, make yourself by mixing 1 tbs vinegar, pour it in a one cup measuring cup, then fill it up with milk. Make buttermilk as much as needed to soak the onion.
Slice the onion thinly and in same thickness. I used a food processor to do this job by using 1 mm blade.
Soak the sliced onion in buttermilk for one hour. This is important step that needed. I tried once without doing this step and just coat the onion with the flour mix, they turned out not crispy.
Drain the sliced onion, then get a handfull of onion and coat it really well with my homemade tempura mix (recipe is on #pib_tempuramix or find "HOMEMADE TEMPURA MIX" on my cookpad).
Heat the frying oil over medium heat until really hot.
Fry onion in batches, stirring as needed, until evenly golden browned. Drain on paper towels and let them cool. Store in an airtight container.