Preheat the oven to 225C/450F/Gas 8.
Heat the clarified butter in a large ovenproof frying pan and layer the potatoes a circle until the base of the pan is covered.
Cover the potatoes with anchovies, making sure that they are skin-side up and that the tail is pointing towards the centre of the pan.
Scatter the majoram over the tart and then layer the tomatoes on top. Season to taste with salt and freshly ground black pepper and place in the oven for 6-8 minutes, or until the potatoes and anchovies are cooked through.
Blanch the basil in a large pan of salted boiling water for one minute, then drain and refresh in iced water.
Squeeze out excess water from the basil with a tea towel. Put the leaves into a blender with the olive oil and blend for two minutes, or until the mixture is smooth and vibrantly green. Then strain through a fine sieve.
To serve, mix the rocket with a little olive oil and place on top of a tart slice. Finally, drizzle with a little basil oil.