Sterilize your bowls and containers
Remove the blossoms from the elderflower heads, discarding as much stem as possible (do not rinse), and place them into a large bowl. Add the honey.
Heat the water in a saucepan or kettle until just simmering, but not boiling. Pour the hot water over the elderflower mixture and use a wooden spoon to stir until the honey dissolves fully. Stir in the lemon slices.
Place a finely woven cloth or tea towel over the bowl and set in a cool, dark place. Let it steep for 2-3 days, checking after the second day. If you’d like it to be stronger, leave until day three.
Once the cordial is finished steeping, strain it through a fine-meshed sieve and pour into sterilized jars or bottles. Keep in the refrigerator for up to a month, or freeze for longer storage.