Combine the sugar and lemon juice, use the coat the apple segments, cover and leave to infuse for approx. 15 mins.
Beat the butter and sugar for approx. 3 mins. using the whisk on a hand mixer. Add the eggs one at a time and continue to mix until the mixture is lighter in colour, season with salt. Stir in the lemon zest and juice. Mix the flour and baking powder and briefly stir in, with the cream. Divide the dough mixture into the prepared moulds. Top with the slices of apple, press down lightly.
Bake for approx. 25 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool on a cooling rack.
Dust the Thurgau apple tarts with icing sugar shortly before serving.