Place beef tongue, onion, garlic, and bay leaf in the slow cooker; generously season with salt. Pour in enough water to cover the mixture.
Cook on low for 8 hours.
Transfer beef tongue to a work surface and let cool slightly. Peel outer layer of skin from beef tongue and remove rough end. Chop the meat into bite-size pieces.
Heat butter in a skillet over medium heat; cook and stir beef tongue meat until tender, 5 to 10 minutes. Season with salt and pepper. Beef tongue is perfect in tacos, wraps, sandwiches, or atop a salad.