Before making the jam sterilize any canning jam jars by running them through the dishwasher or boiling them in a large pot of hot water for 10 minutes.

Add the rhubarb, strawberries and sugar to a large non-reactive bowl and toss to combine. Let sit until the fruit is macerated, about 2 hours.

Add the fruit and sugar mixture with the lemon zest and lemon juice to a large non-reactive saucepot and bring to a boil over medium-high heat. Stir in the vanilla then lower the heat to medium.

Skim any foam off the top and discard. Ladle the jam into the prepared jars, leaving ½-inch of space at the top. Wipe the rims of the jars with a towel then close the jars.

To can the jars add them to a large pot of boiling water with the water covering the top by 1-inch and boil for 10 minutes. Turn the heat off and continue to let the jars sit in the water for an additional 5 minutes.

Use tongs to carefully remove the jars from the water. You should hear the lids suction while in the pot but to test them, remove the ring from the jar and lift the jars by the flat, inner lid. If the lid releases then it is not properly sealed. Unsealed jars should be used within a month and kept in the refrigerator. Sealed jars can be kept in a dark area for up to 6 months.