This bright summer berry sauce is not as sweet as you might imagine. The generous amount of lemon juice cuts nicely through the sugar and the lavender adds a subtle floral note. This sauce is best when made with peak-season blueberries, but you can easily use frozen fruit in the off-seasons. Note that the amount of sugar will vary depending on the sweetness and ripeness of your fruit.
Using a small square of cheese cloth or a disposable tea bag, wrap the lavender up in a bundle.
In a medium saucepan, whisk the sugar with the cornstarch, salt and ¼ cup of water. Add the blueberries, lemon zest, lemon juice and the lavender sachet. Bring to a boil over moderate heat and cook, stirring gently, until the sauce is beginning to thicken, about 5 minutes. Remove from the heat and let cool completely.
Discard the lavender bundle and the strips of lemon zest, then scrape the sauce in a heatproof 1-pint jar. Refrigerate until ready to use. Serve the sauce warm or at room temperature.
MAKE AHEAD: The blueberry sauce can be refrigerated for up to 2 weeks.