Start by toasting the bread. Brush the bread slices with olive oil, place them on a hot griddle pan and toast for about 4 minutes on each side. Set aside.
Next make the white sauce. In a small saucepan melt the butter then add the flour, whisking until a nice thick paste forms. Then reduce the heat to low and add the milk, whisking constantly for about 5 mins. Remove from the heat and stir in the cheese until it has melted and is thoroughly combined. Cover to keep warm.
In a separate small saucepan gently heat the the spoonfuls of Blackcurrant Jam with a dash of water before adding in the blackberries which will start to release their juices.
To serve, top each toast with the Gruyere sauce and divide up the Berry topper. Sprinkle with crushed walnuts and serve immediately.