In a saucepan over medium heat, bring the blueberries, sugar and half the cream to a boil. Remove from the heat and add the lavender. Cover and let steep for about 5 minutes. Strain through a fine mesh sieve. Add the remaining cream. Cover and refrigerate until thoroughly chilled, about 2 hours. Add the lemon juice and whip until soft peaks form. Keep chilled.
With the rack in the middle position, preheat the oven to 180°C (350°F). Line a baking sheet with aluminum foil.
In a 20-cm (8-inch) square cake pan, combine 250 ml (1 cup) sugar and the cornstarch. Add the blueberries and stir to combine. Set aside.
In a food processor, combine the flour, baking soda, salt and remaining sugar. Add the butter and pulse a few seconds at a time until the butter is the size of peas. Add the cream and lemon juice and pulse again until the dough becomes crumbly. Remove the dough from the food processor.
Spread the dough evenly over the blueberries. Sprinkle with large-crystal sugar.
Place the pan on the baking sheet (the sheet will catch any blueberry juices that boil over). Bake for about 70 minutes.
Serve warm or cold with lavender whipped cream or vanilla ice cream.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.