Grease 6 x 150ml pudding basins. Spoon 1tbsp lemon curd into the base of each. Reserve 12 pistachios for garnish, then pulse the rest in a food processor until they're roughly chopped.
To make the sponge, beat the butter, sugar and lemon zest in a bowl with an electric whisk until light and fluffy. Beat in the eggs, one at a time, until well mixed. Sift in the flour, then add the pistachios and lemon juice. Mix until well combined.
Spoon the sponge mixture over the lemon curd. Cover each basin with nonstick baking paper, then with foil, and tie securely with string. Put them in a flat-bottomed steamer over a large pan of boiling water. Cover with a tight fitting lid and steam for 35-40mins, until the puddings have risen and are firm to the touch. Check regularly and top up with boiling water if necessary.
Chop the reserved pistachios. Turn out the puddings, decorate with chopped nuts and serve.