An umeboshi immitation using rhubarb, which is often easier to source. This recipe results in umeboshi paste, not attempting to immitate the shape of real ume. Unlike true umeboshi, this is not fermented and should be kept refrigerated.
Cut rhubarb into 1cm wide slices. Soak in a bowl of water for 20 minutes.
Drain rhubarb. Sprinkle salt over rhubarb and let rest to release water.
Transfer rhubarb to a small saucepan and cook over low until tender.
Remove from heat and transfer to a mortar. Mash into a paste.