For the spring lamb belly fritters, preheat the oven to 150C/300F/Gas 2.
Heat a little oil in a large ovenproof frying pan and brown the lamb breasts. Add the garlic, thyme and the four chopped shallots and lightly colour. Pour in the chicken stock and place in the oven for three hours or until the meat is very tender.
Line a terrine mould with cling film. Remove the meat from the pan. Pull the meat from the bones. Press the meat into the tin. Cover with cling film and place a weight on top to compress the meat. Leave overnight in the fridge.
The next day, make a pickling liquor by boiling two teaspoons of water, the white wine vinegar, sugar and fennel seeds in a saucepan. Pour onto the sliced shallots rings and leave to cool.
For the spinach and anchovy mayonnaise, in a food processor, blend the egg yolks, anchovies, mustard and the pickle liquor from the shallots. Then, with the motor running, slowly add the rapeseed oil in a thin steady stream until it emulsifies into a mayonnaise.
Wash the spinach and pat dry using kitchen paper.
For the spring lamb belly fritters, heat a deep-fat fryer to 180C/350F. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Chop the cooked lamb breast into 2.5x8cm/1x3in rectangles. Coat the lamb in flour and pat off any excess. Dip into the beaten egg then dredge in the breadcrumbs, taking care to completely coat the lamb.
Deep fry the fritters until crisp and golden-brown. Season with salt and pepper when they come out of the fryer.
To serve, dress the spinach leaves with the pickled shallot rings and a little anchovy mayonnaise. Serve with the fritters and the remaining mayonnaise in a dipping bowl on the side.