Slather the yellow mustard all over the surface of the meat on all sides. This will help the rub stick to it. Coat all sides with your rub, massaging it into all of the crevasses.
Set up your smoker to 250℉. Place the pork butt onto the hot smoker and smoke for 2 - 3 hours. Remove it when it's not fully cooked, around 130℉. (You will finish cooking it inside).
Once the pork is cool enough to handle, cut into medium sized chunks.
In a heavy bottomed large pot over medium high heat, add your oil and heat. Once hot, add in the chopped onion. Season with salt and pepper. Cook for a few minutes until onions are slightly translucent. Add garlic and red pepper flakes, stir. Cook for a minute or until fragrant.
Dump in your pork chunks, stirring to incorporate. Add in the broth. Bring to a low boil, cover with a lid, and lower the heat slightly to a simmer. Simmer until pork is cooked and tender, stirring occasionally, about 2 hours.
When the pork is tender, carefully use an immersion blender to blend the pork mixture to a slurry-like consistency. Taste and add salt if necessary. If it's a little too thick, add some more broth about ½ - 1 cup (it should be a little loose to cook the potatoes).
Next, add in your cubed potatoes and the yellow mustard, stirring to combine. Simmer on medium low to cook the potatoes, about 20 minutes.
Serve over rice and enjoy.