Make the pastry by rubbing the chilled butter into the dry ingredients to form a breadcrumb like mixture.
Add enough water to form a dough and wrap in clingfilm and chill for 30 minutes in the fridge.
Grill the pepper until skin blisters, pop into a plastic bag till cool and remove the skin. Cut the flesh into thin strips and set aside.
Scrub mussels and remove beards.
Preheat oven to 190C/375F/Gas 5 and lightly grease tart tins
Roll out the pastry to 2-3mm thick and using a fluter cutter create circles to fit the tart tins, about 5cm in size. Pop them into the tins and prick all over with a fork. Fill with baking beans and bake blind for 8 minutes.
Remove from oven and take out the beans. Put them back into the oven for a further 4 minutes or until the tart shells are lightly golden.
Reduce the oven temperature now to 180C/350F/Gas 4
Heat the oil in a pan and cook the leek for about 10 minutes until soft and begining to brown.
Season with salt and pepper.
In a large pan bring 2.5cm/1" of water to a boil and add 2 tsp of salt, now toss in the mussels and cover the pan. Cook on high for 4 minutes.
Discard any mussels that have not opened and scoop out the mussel meat from the good ones.
Beat the egg and cream together and add to the cooked leeks.
Put a spoonful of the leek mixture on to eact tart case, top with a mussel or two (depending on size) and a criss cross of the yellow pepper.
Beake for 15 minutes or until just firm.
Garnish with parsley