On a large cutting board mash butter, mustard, thyme and salt until well incorporated.
Scrape on to a plate and place in freezer for 10 minutes, this makes rolling the butter easier.
Remove butter from freezer and return to the cutting board. Using your hands, form butter into a roll about the length of the original stick of butter.
Place a large piece of plastic wrap on a clean work surface and transfer your formed butter about 2 inches from the bottom of the wrap. Roll the plastic wrap around the butter, using all the wrap, twisting the ends like a candy wrapper when finished. Fold ends over the butter roll and place in a freezer bag. Freeze overnight.