Preheat the oven to 180C/160C Fan/Gas 4.
Lightly dust a work surface with flour. Roll out the pastry to the thickness of a pound coin, to line a 23cm/9in diameter, 4cm/1½in deep tart tin. Prick the base with a fork but do not trim the pastry.
Cover the pastry with baking paper, fill with baking beans and bake ‘blind’ for 30 minutes, or until it just starts to colour.
Carefully remove the baking beans and paper. (If there are cracks, use leftover pastry or beaten egg to fill them.) Return the pastry case to the oven for 5 minutes. Leave to cool on a baking tray. Trim the pastry.
For the custard, put the eggs, yolks and cream in a bowl and whisk gently, adding a pinch of nutmeg, salt and pepper. Sieve the custard into a pouring jug.
If using dressed crabs, separate the crab meat with your fingers and remove any shell. Flake the white meat and set aside, then spread the dark meat over the pastry case. If using only white meat, spread half of it over the pastry case.
Heat the oil in a frying pan, then quickly fry the spring onions until just softened. Add them to the pastry case.
Stir the white crab meat and Parmesan into the custard, lightly season, then carefully pour it on to the spring onions.
Bake the tart in the centre of the oven for 50 minutes, or until the filling is just set. Leave to cool for a few minutes before serving.
For the guacamole, in a bowl mix all the ingredients well and season. If making in advance, cover the surface tightly with a layer of cling film, then cover the bowl with another layer of cling film and set aside.
Serve slices of the tart with the guacamole in a serving bowl.