Gather the ingredients. Crack the egg into a clean bowl (which has been kept refrigerated) and gently beat with a fork until the yolk and egg whites are barely incorporated.
Combine the water and ice cubes in a cup. Give it a quick swirl with a spoon. Strain the water (you should have one cup or 227g of ice cold water), leaving out the ice cubes. Now add just the ice water to the bowl with the beaten egg.
Sift the all purpose flour and add it to the bowl with the egg and water mixture. Combine it gently with a pair of chopsticks. Take care not to overmix the batter so there are still some floury lumps left. Return the bowl of tempura batter to the fridge and keep refrigerated until ready to deep fry your courgette flowers.
Prepare the stuffing. Combine the feta, garlic, olive oil and mint leaves in a bowl. Gently open the courgette flowers and fill each blossom with a teaspoon of the stuffing. Give the petals a gentle twist to seal it so they don't open during the cooking process.
Fill a wok with 3-5cm of sesame oil and turn on the heat. A second before the oil temperature reaches 180C on a cooking thermometer, coat your courgette flowers in cake flour so the batter will adhere to it, then lightly dip it into the tempura batter. Note that too much batter results in crispy exterior and mushy interior. Next, immediately place the flowers into the pot of oil to cook.
Deep fry the flowers until golden brown, about 2-3 minutes. Occasionally turn the flowers so all sides are browned. Fry them in batches, taking care not to overcrowd the pot as the oil temperature will drop quickly. While frying, keep your eye on the thermometer and ensure the temperature stays between 160C (320F) and 182C (360F). Any higher than 182C will make the tempura flowers too crispy; any lower than 160C and the tempura will absorb too much oil and won't get crispy.
While you're frying between batches, scoop up the excess crumbs in the oil which will burn and turn the oil darker if left in the pot for too long.
Once done, place the tempura on a plate lined with kitchen towel or parchment paper to absorb the excess oil. Crack some pepper on it and season with salt.
Prepare the tempura dipping sauce. If you'd like to make a tempura dipping sauce with dashi soup stock and mirin, Namiko from Just One Cookbook has a nice recipe for it (I have linked to her website here below!). Enjoy your tempura on the spot while it's hot and crispy!