Trim the beef well. Place a large piece of cling film onto a work surface. Mix the salt, sugar and pepper together and place half of it in the middle of the cling film. Top with the beef and place the remaining mixture on top, patting it around to cover all sides. Wrap tightly in the cling film and repeat with another double layer, as it will release a lot of moisture. Place in a container and refrigerate for 24 hours, turning a few times.
Remove the beef from the cling film and rinse under cold water. Pat dry with kitchen paper and slice very thinly.
Top the baguette slices with a slice of beef and a small dollop of the horseradish cream. Sprinkle with the herbs and serve.
To make the horseradish sour cream: Mix the ingredients together and season with salt and pepper.