Start by peeling 2 of the oranges. Using a sharp knife cut the top and bottom, place the orange on one end and slice the skin off downwards all the way around (ensuring you are also removing the white skin around the orange). Cut each orange horizontally into 4 slices. Sprinkle each slice (both sides) with a little sugar (1 tablespoon should be enough for the whole lot).
Using a large non-stick pan heat up 1 teaspoon of butter, place the orange slices in the pan and fry over a medium heat for 1 minute on each side (the butter, sugar and juice will start caramelising and bubbling up). Quickly transfer the oranges along with the sticky juice into a blender, add the orange zest plus juice of the remaining orange (some of the pulp can go in too) and blitz the mixture until very smooth.
In the saucepan heat up the remaining butter, add the flour and whisk together until smooth. Add the orange mixture and bring to the boil whisking all the time. Lower the heat, add the remaining sugar, lemon juice, pinch of salt, stir and simmer for 2 minutes whisking often. Serve warm.
This orange sauce may thicken after a while so just squeeze in a few drops of orange juice to get the consistency you want.