In a bowl, mix together the salt, pepper and garlic. Place your chuck roasts on a baking sheet, lather with yellow mustard and generously season all sides in order to completely cover the roasts. Place them in the fridge, uncovered, for at least 4 hours but ideally overnight.
Preheat your smoker to about 250F. Add some wood chips or wood chunks if desired for extra smoky flavor.
Pull your chuck roasts out and place on the smoker to cook until 165F internal (about 3 hours). Once the chuck roasts have hit temperature, pull them out and place in a large foil bin. Fill into the bin your beef broth, beef tallow and sliced white onions. The liquid in the bin should come up to 50% of the chuck roast’s height, so add more or less if necessary. Tightly cover your bin with foil and place back on the smoker at 300F to smoke until the meat is tender to probe and shreddable (another 3-4 hours). Once the chuck roasts are done, pull them off and let them rest for 20 minutes. After the rest, pull the chuck roasts out and shred them up. (Optional: strain and simmer the au jus down by half for a dipping sauce)
While keeping the slider rolls together, slice your Sliders Rolls in half so there is a top side and bottom side. Add the bottom side into a foil bin or cooking safe skillet. Add a layer of steak Sauce, then start building with quite a bit of pulled beef, sliced cheddar cheese, more steak sauce and then the top part of the buns. In a bowl, melt some butter. Gently spread the melted butter over the top of the rolls then top.
Get your smoker to a medium heat (around 300-325F) for indirect cooking. Add your sliders in the foil bin on to the smoker to cook until they are golden brown & crispy (about 10-12 minutes). Once done, pull the sliders off and let cool for 2-3 minutes before serving.