Toast the coriander, cumin, nigella, and mustard seeds in a small frying pan for 3 minutes.
Grind the toasted seeds into a powder with turmeric and chilli powder.
Whisk flour, ground spices, yeast, salt, sugar, beer, and vinegar into a thick batter.
Let the batter sit for 30 minutes until it begins to bubble.
Heat a deep fat fryer to 190°C.
Coat the crabs in batter and deep-fry for 4 minutes until golden-brown.
Drain crabs on kitchen paper and season with salt.
Simmer garlic in white wine for 5 minutes, then discard the liquid.
Blend egg yolks, vinegar, mustard, and salt in a processor until smooth.
Slowly drizzle in rapeseed oil while blending to create a mayonnaise.
Blend in the coriander, cooked garlic, and lemon juice.
Mix yoghurt, cucumber, shallot, coriander, mint, grated garlic, green chilli, salt, sugar, and lime juice.
Serve crabs with both sauces and garnish with chilli, spring onions, cress, and lime wedges.