Preheat the oven to 150C. Defrost the blackcurrants, then place them along with 75g of sugar and 1 tablespoon of water into a small saucepan. Heat this for 4-5 minutes on a low heat while stirring occasionally. Continue this until the fruit is soft but retains its shape and is sitting in a pool of syrup. Move half the blackcurrants into a sieve with most of the juice, and press down into a small bowl, discarding the solids in the sieve. Set aside this puree and the remains of the blackcurrants left in the saucepan to cool until it is needed.
Place the digestive biscuits into a food processor, or blender and pulse until all the biscuit is crushed. Add the butter and process again until combined. Press the crumb mixture into the base of the tin and place into the fridge until needed later. For the filling, beat the cream cheese gently until smooth, then slowly add the sour cream, the remaining 200g of sugar, eggs, lime juice and zest. The mixture should now be smooth and thick.
Spread the blackcurrants evenly from the pan over the crumb based tin and pour the the cream cheese filling on top. Add drops of the remaining blackcurrant puree and swirl gently with a knife, for a marbled effect. Bake for 1 hour until firm to the touch. Leave to cool at room temperature and refrigerate until ready to serve with remaining blackcurrant sauce.
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