Preheat the oven to 160°C and grease the ovenproof dish well with a little extra reduced fat spread.
Put the flour, reduced fat spread, granulated sweetener, caster sugar, baking powder, eggs and lemon zest in a medium mixing bowl and beat together for 1-2 minutes with an electric whisk (you can use a wooden spoon, but it will take more effort!).
Use a rubber spatula to scrape the mixture from the mixing bowl, into the greased ovenproof dish and level the surface with a knife. Bake in the preheated oven for 20-25 minutes, until risen and golden all over. To test if it's ready, insert a small sharp knife into the centre of the sponge: when the sponge is cooked the knife will come out clean. Leave the sponge in the dish to cool.
Spread the lemon curd evenly over the top of the sponge and cut into 16 squares. Place a small lemon segment on top of each square. Serve alone or with a dollop of custard.