Prep. Preheat oven to 400℉. Grease a baking sheet with cooking spray or oil and set aside.
Cook beef and onion. In a 12-inch skillet, cook the beef for 5-7 minutes over medium heat until browned, breaking into smaller pieces as you cook. Add onion and cook another 5 minutes until the onion becomes softened and looks translucent. Drain excess grease.
Add seasonings. Add chili powder, cumin, oregano, garlic powder, salt, and pepper and stir into the meat mixture to season the beef.
Add rest of ingredients. Stir in tomato sauce, black beans, and the tomatoes and green chilies. Stir in shredded cheese and let it melt, stirring occasionally for 2-3 minutes.
Fill. Wrap tortillas in damp paper towels, place them on a plate, and warm them in the microwave for 20-30 seconds to soften. For smaller tortillas, spread ¾ cup of chimichanga filling down the center of the tortilla, leaving an inch border from the edge. For burrito-size tortillas, spread 1 ¼ cup of filling.
Roll. Roll the filled tortillas up burrito style, folding the ends in and over the filling. Then, roll the tortilla over the filling, tightening as you roll.
Bake. Place on a baking sheet and spray with cooking spray or brush with oil. Place in the oven and bake for 20-25 minutes or until golden brown.
Cool and serve. Rest for 5 minutes after removing from the oven, and then serve immediately with optional toppings.