If you'll be making your own elderflower syrup, start a day or two in advance: Pour water in a pot, add sugars and citric acid, and bring to a boil. Mix well and let all the sugar dissolve. Remove from heat and let cool down. Then, add mint leaves, slices of one lemon and the elderberry flowers - remove as much stem as you can and only use the florets. Mix in, then cover the pot with a cheese cloth or a thin kitchen towel, and let infuse for about a day. Then, drain the syrup and pour it in a bottle. Keep refrigerated and use in a week or so.
To make the panna cotta: add heavy cream and milk to a pot, add sugar and elderflower syrup. Slowly bring to a low boil while stirring and cook on low for about 15 minutes, stirring regularly, until the cream thickens. Remove from heat.
While the cream is cooking, soak gelatin in cold water, according to package instructions (or use agar agar for a vegetarian version). Once soaked, squeeze the gelatin to drain liquids, then add it to the hot cream and stir it in, whisking thoroughly until the gelatin dissolves. Pour the cream in jars or glasses and let it cool down before storing in the fridge. Keep refrigerated for at least 4 hours, or overnight, until the cream firms up.
Prepare decoration: Wash and dry strawberries, then cut in half or in slices. Sprinkle with sugar and add 1-2 tablespoon of elderflower syrup. Let macerate for about 15-30 minutes, then add strawberries on top of the panna cotta.
Finish your decoration with a couple of elderberry florets and some fresh mint leaves and serve.