For the spring rolls, place the crab meat into a bowl with the spring onions, cucumber, lime juice and crème fraiche. Mix until well combined and season with salt and freshly ground black pepper.
Place a spring roll wrapper onto a chopping board lengthways. Place a tablespoon of the crab mixture 3cm/1¼in from the bottom edge of the wrapper. Fold the bottom lip over, then the sides in and roll up into a cylinder. Seal the end edge with beaten egg. Repeat with the remaining spring roll wrappers and filling.
Heat the oil in a saucepan until a breadcrumb sizzles and turns brown when dropped into it. Alternatively, use an electric deep-fat fryer heated to 180C/350F. (CAUTION: Hot oil can be dangerous. Do not leave unattended.)
Carefully lower the spring rolls in small batches into the oil and deep-fry for 2-3 minutes, or until golden-brown. Remove with a slotted spoon and drain on kitchen paper.
For the crab cakes, place the crab, potato and coriander into a bowl and mix until well combined. Season with salt and freshly ground black pepper. Add lemon juice, to taste.
Take a golf-ball sized piece of the crab mixture and form into a flat pattie.
Heat a frying pan until hot and add the olive oil. Fry the patties on each side until golden-brown and cooked through.
Dress the watercress with a little extra virgin olive oil and lemon juice.
Pile the spring rolls and crab cakes onto serving plates and finish with a handful of watercress.