I have no idea where the recipe began, or even how the recipe got the name Mississippi attached to it, because best I can figure, it hasn't got a thing to do with Mississippi - unless somebody who threw it together one day just happened to be from Mississippi.
Using the tip of a knife to cut multiple small slits in areas all over the meat and inserting slivers of garlic into each cut. Season roast on both sides with pepper. Heat 1 tablespoon of fat or oil in a large skillet and brown roast on both sides.
Add the celery chunks to the bottom of a 6 quart slow cooker and place roast on top. Add onion to skillet and cook for 3 minutes. Transfer to top of the roast. Meanwhile whisk together the ranch dressing,gravy mix,thyme and rosemary. Add the Coke and whisk in until well blended. Set aside.
Add the bay leaf,whole jalapeno,peperoncini and giardiniera to the top of the roast and pour the Coke mixture over the top. Cover and cook on low for 7-8 hours,or on high 4-5 hours.
Total time will be dependent on size of roast.
Use a long fish spatula to remove the roast and loosely tent with foil to keep warm. Skim any excess fat off the top of the juices and carefully transfer the contents of the crockpot to a saucepan. Stir in the tablespoon of butter. Make cornstarch with just enough water to dissolve the cornstarch.
Whisk into the gravy, bring up to a boil, reduce and simmer until thickened.
Cook's Notes: Substitute 1 cup of water,low sodium beef broth,wine or beer,in place of the Coke. If you're adventurous,eliminate the liquid and add that whole stick of unsalted butter on top.
Golden peperoncini are a milder, sweet pepper with just a hint of heat but to make it milder leave them out,along with the jalapeno and giardiniera. Puree the veggies and add those to the gravy for extra flavor.