Put the blueberries, balsamic vinegar and rosemary in a small saucepan. Heat gently until just bubbling, then remove from the heat and set aside. Meanwhile, heat the oil in a large frying pan over a medium heat.
Add the garlic, cavolo nero, 1 tbsp water and the salt and fry for about 3-4 minutes, turning regularly, until wilted; tip onto a plate and set aside. Leave the pan with the garlic off the heat, ready for the sandwiches.
Spread one side of each slice of bread with butter – these will be the outsides of the sandwiches. Spread two of the inside slices with mustard, then layer up with the cavolo nero, brie and ﬁnally the balsamic blueberries (discarding the rosemary).
Put the remaining slices of sourdough on top (with the butter on the outside) and press down lightly.
Return the frying pan to a medium-high heat. Add the sandwiches, cover with a sheet of parchment, then top with another frying pan, weighing it down with 2 cans of beans.
Fry for 3 minutes, then carefully turn the sandwiches and fry on the other side for 3 minutes. Remove the sandwiches from the pan, cool for a minute, then halve and serve.