Peel and coarsely grate the carrots. Chop walnuts coarsely. Separate eggs. Beat the egg yolks, 75 g sugar and lemon zest with the whisks of the hand mixer until thick and frothy. Fold in grated carrots. Mix flour, almonds and walnuts and stir into the egg cream. Beat egg whites until stiff, add salt and 150 g sugar. Fold into the mixture.
Put the mixture into a greased square springform pan (24 x 24 cm) and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 50 minutes. Remove from the oven and let it cool down on a cake rack.
Remove the cake from the mould. Whip the mascarpone, cream cheese, icing sugar and lime zest with the whisk of the hand mixer. Pour onto the cake and spread. Serve sprinkled with lime zests.