Add the rhubarb, sugar, water and zest and juice from both lemons to a dutch oven or heavy-bottomed pot. Put the pot on medium low heat.
Give the mixture a gentle stir as they begin cooking. Then let it sit for 45 minutes, coming back to stir occasionally so the bottom doesn't scorch. It's ok to have it at a very light simmer, but don't let it go any higher. Stir in the salt halfway through, if you prefer it in the jam.
After 45 minutes, you'll know when the jam is done when you can scrape the bottom of the pot and the jam is thick enough that you can see the bottom. Remove it from the heat and let it cool before storing.
Store in a jar or glass container and keep in the fridge. It will last for up to 2 weeks.