Heat a large skillet over medium-high heat. Add olive oil, tomatoes, onions, garlic and salt. Saute for 8 to 10 minutes, until onions become translucent and tomatoes begin to break down.
Add diced green chiles, chili powder, oregano and cumin. Mix together and cook for another 5 minutes.
Remove skillet from heat and add in shredded chicken. Toss together to coat. Taste and season with more salt, if necessary.
Preheat oven to 400°F.
Assemble the chimichangas: Place about 1/3 cup of the chicken mixture into the center of each tortilla and top with 3 heaping tablespoons of shredded cheddar cheese. Fold the opposite sides of the tortilla over the filling and roll it into a burrito shape. Place it seam-side down onto a large baking sheet covered with parchment paper.
Brush each chimichanga with melted butter and bake for 25 minutes, or until the top is golden brown and crispy.
Serve with your toppings like sour cream guacamole, salsa and enjoy.
To save 20 minutes: Use 1/2 to 3/4 cup of your favorite chunky salsa instead of making your own. Just toss it together with the shredded chicken and skip to step 4 of the instructions.To freeze: Let chimichangas cool completely. Then, place them in a large freezer-safe storage bag. Close and freeze.To reheat: Place chimichangas in 350°F oven for 10 to 15 minutes, until completely heated through. You can also reheat them in the microwave in 1 minute intervals until heated through.