Wash and brush the quinces to clean them. Using a ceramic knife (quinces are very tough), remove the cores and core housings.
Dice the quinces and place them in a large saucepan.
Score the lemongrass stalks with a knife to release their flavor and add them to the pan. Cut the fresh ginger slices into small pieces and add them. Remove the seeds from the fresh chili peppers, cut them into small pieces, and add them to the pan.
Peel and quarter the onions and cut the leek into pieces. Add these along with the sugar, curry paste, salt, coriander powder, water, and apple cider vinegar (a good shot) to the pan.
Bring to a simmer over medium-low heat and cook gently for 45 minutes, stirring occasionally, until the quinces are very soft.
Remove and discard the lemongrass stalks. Using an immersion blender, process everything into a smooth mousse. Taste and season again, possibly adding lemon juice to adjust flavor.
Immediately pour the hot mousse into jars that have been rinsed with hot water (around 80-100°C / 176-212°F). Close them tightly and turn the jars upside down.