For this decadent toastie preheat a skillet or heavy-duty griddle. In another small frying pan melt half your butter and add in your chopped mushrooms, rosemary and truffle sauce. Sauté for 5-10 minutes and then start building your toastie layers.
Layer some kraut on the bottom, followed by a generous handful of rocket. Next your truffle mushrooms and top with a big slice of ripe brie.
Finally brush the remaining butter onto the outer sourdough and place into your hot skillet. Place another heavy pan or an iron grill press on top.
Cook for 4-5 mins and turn at least once during cooking.