In a medium bowl, combine the chopped plums, sugar, lemon juice and flavour (either a cinnamon stick OR the Earl Grey tea leaves wrapped in a cheesecloth to form a make-shift tea bag). Stir everything together and let rest on the counter for 6 to 8 hours, stirring every so often. The juices from the plums will form a syrup with the sugar as it dissolves.
At the end of the day, stir once more and then cover with plastic wrap and chill overnight.
The next day, transfer the macerated plum mixture to a large pot and heat on medium–high to bring it to a boil, stirring often.
Skim off any foam as needed and continue to boil/stir until the jam has thickened (I boiled one batch to 215ºF and the other to 217ºF). The jam is probably thick enough when it starts spitting at you.
Remove the cinnamon stick or the Earl Grey tea bag from the pot. Transfer the hot jam to hot, sterilized 250-mL mason jars using your handy canning tool kit available on Amazon, leaving ¼" headspace. Cover with clean, sterilized snap lids. If a jar isn't filled full, put it in the fridge to enjoy right away.
Process in a large boiling water batch for 5 minutes, then leave them another 5 minutes, still in the canning pot but off the heat before transferring the jars to a clean towel on the counter.
Let the jars rest overnight before labelling and storing them.