Cook the pasta in salted water according to the instructions on the packet.
In the meantime wash the lemons. Rub the peel, squeeze out the juice. Bring the juice, broth and mascarpone to the boil while stirring. Stir in the sauce thickener and lemon zest. Season to taste with salt, pepper and sugar.
Pour the sauce over the drained spaghetti, stir and serve hot.