Whisk the eggs, sugar, milk and vanilla together. Mix the fruits and put the mixture between the brioche slices, pressing them firmly together so that they crush the fruit slightly.
Put the sandwiches in a tray, pour over the egg mixture and leave to stand for about 2-3 minutes. Heat the butter in a preferably non-stick frying pan until it is foaming, carefully remove the sandwiches from the tray with a fish slice and cook for about 2 minutes on each side until golden. Cut the sandwiches in half, transfer on to serving plates and serve with the crème fraîche.