To make the spice rub, combine chili powder, cumin, oregano and salt in a small bowl.
Sprinkle the spice rub over the chuck roast, rubbing in thoroughly on all sides.
Heat canola oil in a Dutch oven or large pot over medium high heat. Add chuck roast and sear both sides until browned, about 2 minutes per side. Add orange juice. Cover and roast until tender and falling apart, about 3-4 hours.
Remove the chuck roast from the Dutch oven and shred the meat before returning to the pot with the juices.
Heat olive oil in a medium skillet over medium high heat. Add onion, bell pepper and jalapeno and cook until tender and golden, about 5-6 minutes.
Preheat oven to 475 degrees F. Lightly coat a baking sheet or pizza pan with olive oil.
Working on a surface that has been sprinkled with semolina or cornmeal, roll out pizza dough into a thin rectangle. Transfer to prepared baking sheet or pizza pan.
Spread tomatillo salsa over the top, leaving a 1-inch border. Top with mozzarella and onion mixture.
Place into oven and bake for 12-15 minutes, or until the crust is crisp and brown on the bottom and the cheese is melted on top.
Top with shredded carnitas, chipotle avocado mayonnaise, green onions and avocado.