For the blueberry syrup: Add the blueberries, sugar and lavender to a 3-quart saucepan and stir briskly, mashing some of the blueberries. Let sit for 30 minutes at room temperature.
Add the lemon juice and 3/4 cup water to the saucepan and bring to a boil over medium heat, stirring occasionally. Reduce the heat to low and simmer, uncovered, 15 minutes.
Run the blueberry mixture through a fine-mesh sieve set over a bowl or container. Using the back of a wooden spoon, mash the mixture against the sieve to ensure you get all the syrup. Discard the leftover contents in the sieve. Let the syrup cool to room temperature and chill in the refrigerator before using.
For the blueberry soda: Fill a 16-ounce glass halfway with ice, add 1/4 cup blueberry syrup, top with seltzer and stir gently to combine.
Frozen blueberries can be used in place of fresh. The recipe makes approximately 4 cups of syrup. The syrup will last up to 2 weeks covered and refrigerated.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.