In a small mixing bowl, whisk together cayenne pepper, chili powder, salt, cumin, onion powder and garlic powder.
Take the spice mixture and rub onto the pork shoulder roast. Place the roast in a slow cooker, and add any reserved spice rub. Cook on a low setting for 6-10 hoΩrs, or until the pork is so tender it falls apart.
Remove the roast from the slow cooker and place on a cutting board. Use two forks to pull meat apart into bite-sized shreds.
Place the pork shreds back into the slow cooker and toss to coat with the juice from the roast.
Serve over tostada and top with crumbled feta and diced avocado.
To make tostadas simply oven grill or deep/shallow fry small corn or flour tortillas until crispy and drain on kitchen paper.