Mix the chili powder, cumin, oregano & salt in a small bowl. Rub the spice mixture on both sides of the roast.
Place roast in slow cooker on low. Pour juice into slow cooker and let roast for 4-6 hours or until meat starts to fall apart. You can also place in 300 degree oven for about 4 hours. When done, use 2 forks to shred the meat, set aside.
Preheat oven to 450 degrees (preheat for about an hour with baking stone or baking sheet inside)
Saute the onions and peppers in a skillet with about 1 tablespoon of butter, until slightly caramelized and soft. Set aside.
Roll out pizza dough. Assemble pizza on parchment paper dusted with corn meal.
To assemble, spread salsa verde thinly all over dough. Top with mozzarella, shredded pork, and then sauteed onions and peppers. Sprinkle with 1/2 cup cheddar cheese.
Slide pizza onto hot baking sheet or stone. Bake pizza for 12 – 14 minutes, sliding parchment paper away halfway through.
Sprinkle with sliced green onions and slice into pieces. Serve with sour cream.