Place beef tongue, onion, garlic, and bay leaf in a slow cooker; generously season with salt. Pour in enough water to cover beef mixture.

Cook on low for 8 hours.

Transfer beef tongue to a work surface and cool slightly. Peel outer layer of skin from beef tongue and remove rough end.

Chop meat into bite-sized pieces.

Heat butter in a skillet over medium heat; cook and stir chopped meat until tender, 5 to 10 minutes. Season with salt and pepper.