In a saucepan, combine the diced quinces, sugar, lemon juice, and cinnamon. Add 100 ml of water to prevent sticking. Cook until the quinces soften, then blend into a smooth puree. Let it cool completely. Whip the cream and gently fold it into the quince puree using a spatula. Distribute the mousse into glasses or bowls and chill in the refrigerator for 1-2 hours before serving.