Place the onion cut-side down and root-side up. Starting 1.5cm from the root, make a downward cut all the way through to the board. Repeat to make four evenly spaced cuts around the onion and make 3 cuts in between each quarter.
Flip the onion over and gently spread the “petals” apart. Coat it with the Prima Tempura Batter Mix and place in freezer for about 30 minutes.
Mix batter ingredients and whisk gently until well-combined.
Dip the onion into the batter and drain off the excess batter.
Heat oil to 180°C. Carefully lower the onion into the oil, cut-side down, and fry for about 3 minutes. Flip the onion over and fry for another 3 minutes or until golden brown.