Roasting fruit imparts a deeper flavor to your berries—if you're loathe to turn on your oven in the heat of summer, fresh berries will work just as well. Mascarpone and Greek yogurt both work beautifully in place of the crème fraîche. —Posie (Harwood) Brien
Preheat the oven to 350 degrees. Spread the berries on a parchment-lined baking sheet and sprinkle with the sugar. Add the thyme, if using.
Bake the berries of 20-30 minutes, or until jammy and bubbling with juice. Set aside to cool down.
In a large bowl, whisk together the milk, cream, sweetened condensed milk, lemon juice, lemon zest, and salt.
Add the milk mixture to your ice cream maker and freeze according to the manufacturer's instructions. When it's almost fully frozen but still has a couple minutes to go, add the crème fraîche and the roasted blueberries. I like to add them at the end so they are still streaky, but if you prefer a more uniform ice cream, you can add them earlier.
Transfer to a freezer-safe dish (I use a parchment-lined metal loaf pan) and freeze until firm.