Wash and cut the rhubarb to about 0.4 inch (1 cm) wide.
Soak them in a bowl of water for 20 minutes. *2
Drain the soaking water and sprinkle salt over the rhubarb.
Leave it for 20 minutes in order to withdraw some of the water.
Place them (with the water that comes out) in a small saucepan and cook over low heat for 7 -10 minutes.
Turn the heat off and transfer it to a Japanese mortar. Mash it in the mortar. *3
Keep it in a sterilised jar in the fridge. *4
Video
Notes
*1 Pick red and thin rhubarb to create a real Umeboshi like paste. *2 This step is for removing the astringent taste of rhubarbs. Soak for no longer than 20 minutes. Otherwise it may lose some acidity. *3 If you don’t have a Japanese mortar, just mash it in the saucepan with a fork.*4 It lasts for about 2 weeks in fridge.