Macerate: Clean and chop the rhubarb. Place in a bowl, add sugar, both types of ginger and lemon juice. Cover with a towel and let stand for about 2 hours.
Preparations: Sterilize the jars, set aside, upside-down on a clean kitchen towel. Place a saucer in the freezer; you will need it to see if the jam is set.
Cook: Pour the contents of the bowl in a large pot and bring to a boil while often stirring to help dissolve the sugar. Once the mixture comes to a full boil, continue cooking while stirring until the rhubarb is tender and the jam is set. It should take between 12 and 20 minutes, depending mainly on the size of the pot and the heat. The wider the pot, the faster the jam will set.
Check: Remove from the heat. Drop a few drops of the jam on the saucer you had in the freezer. Leave them for about 20-30 seconds, then push with your finger; the jam should wrinkle. If it doesn’t, cook it for about 2 more minutes than check again.
Store: Pour the jam into the sterilized jars through a jam funnel and seal. Leave to cool entirely without moving.