Very gently rinse and drain the rose petals and place them in a large bowl with 200 grams of the sugar and the lemon juice. With your hands, massage the rose petals with this mixture until you reduce the petals to a sort of “paste.” The petals should remain whole, not torn, but with the sugar and lemon they will release colour, perfume and wilt.
In the meantime, add the rest of the sugar to the water and heat in a large saucepan until the sugar dissolves. Add the rose petals and bring to the boil. Allow to boil until the syrup thickens and the petals no longer float (about 30 minutes). Remember this is a jam made from flowers, not fruit pulp! It won’t be jammy, but more a lovely silky syrup. Keep an eye on it, and stir every now and then.