Mississippi pot roast: add all Mississippi pot roast ingredients to a slow cooker and cook on low for 8 hours. If you need it cooked a bit faster, you can cook it on high for 5 to 6 hours.
After a long stay in the slow cooker, the meat should be very tender, and it should have created a large amount of au jus. Allow the pot roast to sit in the slow cooker, turned off with the lid on for 30 minutes or so, and then attempt to skim off any fat from the top of the surface of the liquid.
I typically let the beef cool and store in the refrigerator and reheat it for a sandwich with the Sandwich assembly instructions below, but you can serve it immediately if you are eating now. If not eating now, keep the beef in a sealed container for no more than 4 or 5 days. You can freeze cooked pot roast for longer storage if needed.
Crispy fried onion strings: cut an onion into 4 quarters and slice thin slices off the quarter pieces, making thin strips of onion.
Add those thin strips of onion to a medium-sized bowl along with the buttermilk. You just need enough buttermilk so that the onion is submerged. Mix both together and add to the fridge to let the onion soak for 30 minutes to an hour.
In another medium bowl add your all-purpose flour, salt, garlic powder, and ground black pepper. Mix thoroughly.
After 30 minutes or so, remove the bowl with your onions and buttermilk from the fridge and place it beside the bowl with the flour.
Add at least 1 inch of vegetable, peanut, or canola oil to a large pan or pot and place it over medium-high heat. While your oil is heating you can start dredging your onion pieces. grab a handful of onions in the buttermilk and shake off most of the liquid. Add to the bowl with the flour mixture and coat the onions well. Leave the onion pieces in the bowl with the flour until the oil is hot enough.
When the oil has reached 350 degrees F (176 C) or if you don't have a thermometer, when tiny pieces of flour sprinkled over the oil start to sizzle upon entry, you are ready to start frying.
Carefully remove some onions from the flour mixture and shake off excess flour. Add to the hot oil carefully and fry in batches for 2 to 3 minutes or until the onions turn a nice golden-brown color.
Once any onions are cooked, move them carefully to a cooling rack or paper towels to rest while you fry the rest of your onions.
Sandwich assembly: in a pan on the stove over medium heat, add a 1/2 to a 3/4 cup of beef. Make sure to grab a pepperoncini or two and remove the stems. The stems are edible, but I still usually will remove them.
Cook the beef for 2 minutes with an occasional stir to move the meat around. Add cheese slices on top of the beef and cover the pan. Cook everything for 2 additional minutes in the covered pan until the cheese has melted.
Add all the meat and cheese to a sliced sandwich roll. Top with a few crispy fried onion strings and serve.