Preheat oven to 150 C / Gas 3. Grease a 23cm tart tin with a removable base.
Beat softened butter and icing sugar together in a bowl until light and fluffy. Beat in the flour gradually until well mixed. Press the mixture into the bottom and up the sides of prepared tart tin. Bake the base in preheated oven for 30 to 35 minutes or until golden brown. Allow the base to cool in the tin on a wire rack.
Melt the white chocolate in the microwave or over a double boiler. Mix in the mascarpone cheese until smooth. Stir in the lemon zest.
Once the base has cooled, spread the white chocolate mixture evenly over the bottom. Chill the tart for 30 minutes in the fridge before decorating with fruit.
Arrange the fruit in circles over the filling, starting at the outer edge with raspberries, overlapped with strawberry slices, then slices of pear and a pile of blueberries in the middle. Store in the fridge until ready