Add oil to a medium sized saucepan set over medium heat. Add onion and garlic and sauté until fragrant, about 5 minutes. Make sure to stir the mixture frequently and watch the garlic because it can burn easily.
Add tomato sauce, tomato paste, maple syrup, apple cider vinegar, tamari, mustard and molasses (if using) into the pan. Stir to combine.
Add in seasonings and spices including the salt, pepper, chili powder, paprika, cayenne pepper, and cinnamon. Stir to combine.
Allow mixture to come to a simmer over medium-low heat and cook for about 12-15 minutes, stirring frequently. Just be careful because if the heat is too high, the mixture will start to boil and splatter.
Place pork shoulder in a slow cooker. Pour sauce over top. Cook for 8-10 hours on low or 4-5 hours on high.
Shred the pork using two forks (you may wish to transfer the pork to a separate plate) and remove any visible fat. Add the pork back to the sauce and toss to combine. Serve warm.