Place all ingredients in the bowl of an electric mixer. Using the paddle attachment, beat on high speed until all ingredients are well incorporated, scraping sides of the bowl occasionally to ensure all ingredients are combined.
Spread the boursin cheese mixture evenly over six slices of the bread. Arrange the cucumber slices on each piece of bread, making sure to cover the entire surface of the bread. Spread divide the red onions evenly atop the cucumber slices and top with the remaining slices of the bread. Cut the crusts from the sandwiches and cut each sandwich into 3 rectangles. Cover well and refrigerate until needed.