Place the pork shoulder in the crock of a slow cooker. Season the pork with garlic powder, oregano, salt, and pepper. Add the chopped onion and cover.
Cook on low for 8 hours or on high for about 6. The meat should be fall-apart tender. Save the juices for chili, stew, or soup.
Preheat the oven to 400° F.
Remove the meat from the pot and shred it/chop it into chunks. Place it in a baking dish and bake for about ten minutes to crisp the edges just a bit and brown the meat.
Warm the tostada shells according to package instructions.
Heat the beans in a small sauce pot.
Assemble tostadas by layering tostada shell, beans, carnitas, and other toppings.
To freeze: the shredded meat can be frozen for a later date. Simply cool and store in an airtight container. Chill completely before freezing. To serve: thaw completely in the refrigerator. Reheat and serve.Note: the cooked pork is very versatile and can be used in tacos, soups, stews, chili, and sandwiches.Promptly store leftovers in an airtight container in the refrigerator, for up to 4 days.Nutritional values are approximate and based on 1 tostada.