Cut up Vidalia onion. Add the onion & olive oil to the slow cooker on high heat. This should cook for approx 20-30 minutes by itself before adding the roast.
Check to see if the roast will fit into your slow cooker. If the roast will not easily fit, cut the roast in half. Mix the salt and meat rub (or any combination of your favorite herbs). Rub the mixture over the roast.
Combine the Worcestershire sauce, ketchup, garlic and Vidalia onion mustard together. Note: If Vidalia onion mustard is not available, use 1/2 cup Vidalia onion salad dressing & 1 tbsp dijon mustard.
Place the roast in the slow cooker. Pour the sauce mixture over the roast.
Pour in enough water to cover the roast. Set the slow cooker to the "Low Heat" setting. Cook for 6-9 hours. You will know it is done when your entire home smells delicious and you can't wait anymore.