In a small bowl stir together cumin, coriander, salt, onion powder, and garlic powder. Rub mixture all over pork roast.
Place onions in a slow cooker and pour salsa verde over onions. Place pork in slow cooker and cover. Cook on low heat until very tender, 6-8 hours.
In a blender combine avocado, jalapeño, lime juice, oil, and salt. Blend until very smooth, adding up to ½ cup water to reach a smooth, pourable consistency.
Shred pork and toss with cooked onions and enough sauce to moisten. Serve with tortillas, avocado cream, and cilantro.
