Slow Cooker Pork Roast

Combine brown sugar, garlic powder, onion powder, smoked paprika, ground mustard, cayenne pepper, black pepper, and kosher salt in a small bowl. Mix together and set aside.
Pat the pork butt roast dry with a paper towel and rub mustard all over. Sprinkle heavily with the season seasoning blend, ensuring the entire roast is covered. There will be a lot of seasoning leftover, don't worry, we're going to use it all! (optional: place onto a wire rack and place into the refrigerator to dry brine for 24 hours - this will provide the maximum flavor)
Place a large skillet onto the stove over medium-high heat. Add olive oil and place the roast into the skillet, searing on all sides for about 5 minutes, until a nice crust develops.
Add sliced yellow onion, and diced garlic into the bottom of the slow cooker, then add the remaining seasoning mixture on top. Toss together to coat, then pour in the chicken broth and worcestershire sauce. Place the seared roast into the slow cooker and top with butter.
Cover with the lid and cook on low heat for 8 hours, until tender and falling apart. (for a more sliceable roast, cook on low for 5-6 hours).
Optional Finish - For crispy edges, spread shredded pork on a sheet pan and broil for a few minutes. Drizzle with some of the juices before serving.