1 Rinse a large jar or mixed-size jars (for a total of 900 ml)(approximately 30 ounces total capacity) together with matching lids in boiling water and leave upside down to drain on a kitchen towel. Gently rinse blackberries in a strainer and drain.
2 Rinse apples, peel, quarter and core.
3 Cut apple quarters in half lengthwise and cut crosswise into slices.
4 Cut large blackberries in half if necessary. Weigh 750 grams (approximately 26 ounces) of blackberries and apples and mix with jam sugar in a large pot.
5 Bring mixture to a boil over high heat while stirring and cook for an additional 3 minutes, stirring constantly.
6 Fold in lavender flowers, return to a boil and immediately pour into prepared jars. Close lids, invert jars, and let stand for 5 minutes upside down. Turn jars over again and allow to cool. (The spread stores well sealed and refrigerated for about 6 months.)
Tip: To preserve the spread for longer, you will need to process the jars according to manufacturer's instructions, or visit http://nchfp.uga.edu/.