To make harissa mayonnaise, carefully roast the spices in a pan until fragrant. Place in a mortar and finely crush using a pestle. Heat oil in a heavy-based pan and add garlic,salt and capsicum. Sweat,uncovered,over a very low heat for about 2 hours or until soft. Stir often to prevent sticking.
Increase the heat,add palm sugar and cook,stirring,for about 5 minutes until caramelised. The mixture should go a reddish brown. Add fish sauce,roasted spices and chilli powder,then cook,stirring,for 2 minutes. Cool mixture,add lemon juice,then blend until smooth. Whisk the harissa into the mayonnaise and check seasoning.
In a deep fryer or a saucepan,heat some oil to about 180°C. Season the flour with salt and pepper. Toss prawns in seasoned flour,shaking off excess,then deep-fry in batches until pink,crispy and cooked through,about 2-3 minutes. Remove with a slotted spoon and drain on paper towel.
Serve on a platter with lemon wedges and harissa mayonnaise on the side.
Photography by William Meppem. Food styling by Hannah Meppem. Food preparation by Nick Banbury