I simmered it for about 20 minutes....you can do this for far less time if skipping the elderberries. Just soften the fruit. Elderberries are poisonous unless fully cooked, so I made a sauce instead of chunky fruit.
When it cooled, I added liquid stevia to sweeten it. If this is still too tart for you, add a bit of raw wildflower honey along with the stevia. Skip the white sugar. It has no redeeming value. I use it only for fermenting kombucha and wine, since it is completely taken up by the microbes and the yeasts.
Once it was completely cooled, I layered the sauce with chevre (yogurt or strained yogurt would be great, too) to make some grab-n-go snacks for the next few busy days. I put the rest into a quart jar to have with chocolate black bean cake. It would also make a great sauce for pancakes (made with coconut flour for all you low-carbers)...the possibilities are endless.
Next, I chopped the rest of the day's harvest and froze it in a ziploc freezer bag. I'll add to this bag, and start more bags, as the rhubarb continues to send out leaves. The earliest harvests are more tender and less tart, so at some point I will stop picking them for the year. Oh, and remember, the actual leaves of rhubarb are poisonous, so don't feed them to your family or your livestock. I layer them around the plants as a mild weed block.