For instructions on how to prepare quince, please see the text above.
Cut quince into 3 cm pieces and place in a cooking pot along with the 4 cups of water, the cinnamon stick, and lemon geranium if using.
Simmer covered over moderate heat until the fruit feels soft and tender when you run a knife through it.
Drain the water from the pot (you can use it in jellies and jams). Remove the cinnamon stick and lemon geranium.
Pulse the quince with an immersion blender. Or use a high-speed blender.
Add the sugar to the cooking pot with the cooked quince. Add also the grape or pomegranate molasses, if using.
Cook, stirring with a wooden spoon over medium to medium-low heat for about an hour. It takes time to thicken and turn into a paste. The temperature should read 210°C / 410°F for a candied, jelly-like texture. The longer you cook from there, the harder and more candy-like it will become. Test by adding a small drop with a teaspoon onto a cool plate so you can see how firm it becomes as it cools.
Brush a 9 x 13-inch (23 x 33 cm) pan with olive oil. Transfer the quince paste and smooth it with a spoon. Cut and divide into pieces. Press a blanched almond in the middle for decoration (optional).
Cool the quince paste for a couple of hours at room temperature until set and firm. Don't cover the pan, let it air-dry. If it's not as firm as desired, you can bake it at 100°C / 210°F until firm.
Once cooled, you can decorate it with ground almonds or cinnamon sugar. Take each piece and roll it into the desired coating.