Combine the first 6 spice ingredients into a small bowl. Cut the pork into 3-4 pieces and coat with the spice mixture.
Preheat pressure cooker on sauté or brown setting. Add olive oil and brown the pork on all sides. Remove pieces and add 1 cup of salsa. Place the pork on top of the salsa and secure the pressure cooker lid. Set for high pressure for 40 minutes.
While the pork is cooking, whisk together the red wine vinegar, water, sugar and salt in a bowl. Add the red onion, ensuring they are covered in the liquid and set aside.
When the pressure cooker is done, allow it to naturally release for approximately 10 minutes. At that time manually release the remaining pressure and carefully remove the lid. Remove the pork and shred using 2 forks.
Add the pork back into the pressure cooker pot and coat the shredded pork in the sauce.
Line the tostada shells on a serving platter. Spread the top of each tostada shell with a few tablespoons of the butternut squash. Evenly distribute the shredded pork on top of the pork.
Remove the onions from the liquid and sprinkle on top of pork. Top with black beans, cheese, avocado, fresh cilantro and sour cream if desired and serve hot.