Rinse the fruit (of course). Pit and quarter the apricots but don’t bother peeling them. Pit the cherries and cut them in half. Put both in a non-reactive pot (I use this 4-qt one) with the lemon juice and sugar. Combine gently, and let stand while you start to get the jars ready.
Speedy processing hint: start your big pot of water to boil NOW. By the time the jars are ready, the jam will be ready to put in them.If you use a steam canner, just set it up and start the water heating; keep it warm but not boiling.
Meanwhile, prepare jars and any other equipment. Make sure your jars are sparkling clean. Wash the lids in warm water and set aside (see new procedures for jar lids!) Keep the bands handy. Have an extra small jar, or custard cup, clean and ready for any excess jam. Put a couple of small saucers in the freezer to chill so they’ll be ready for testing the jam.
Cook the jam: add the butter (it reduces foaming), and bring to a boil, stirring occasionally. Reduce heat somewhat and simmer it until the chunks of fruit are thoroughly cooked. Blitz the preserves with an immersion blender – this will create a uniform texture, and will help the jam cook faster.
Return the preserves to the heat, and continue cooking, stirring often, until it bubbles and looks quite thick. I like to test for doneness by putting a spoonful on a saucer, and chilling it in the refrigerator for 2 minutes. After that time, if I can drag my finger through the jam, and leave a clear track that doesn’t fill in again, it’s ready. In my kitchen, this took about 10 minutes more.
When the jam is ready, remove the pot from the heat. Ladle jam into prepared jars, wipe rims, and apply lids and bands. Anything that won’t fit in your four jars can go into the clean custard cup and chill it in the fridge.
Wipe the jar rims, apply lids and bands, then process for 10 minutes in a boiling water bath. Cool jars, label, and store in a dark cool place.