Fill a large pot just under halfway with vegetable oil and set over a medium heat, bring to 180°C.
Trim any woody ends from the broccoli and halve any large stalks lengthways.
Whisk together the rice and plain flours along with the baking powder and caster sugar. Gradually whisk in the sparkling water until you have a loose batter that's the texture of double cream.
Lightly dust the broccoli with rice flour, dunk briefly in the batter and carefully lower into the oil.
Fry for 2-3 mins until golden brown and crunchy. Carefully transfer to a lined baking tray or rack and season with salt. Repeat with the remaining broccoli.
Meanwhile, grate the garlic into a bowl and beat in the 'nduja with a big squeeze of lemon juice. Stir in the mayo and season with salt.
Serve the hot, crispy broccoli with lemon wedges and plenty of 'nduja aioli.