Sauté the chopped onion, garlic, and mushrooms (if using) in olive oil until onion is soft.
Add the chopped chilies, flatten mixture in bottom of pan and sauté, not stirring, until the onions begin to caramelize a little, then remove from heat.
In a large bowl, mix remaining ingredients except tortillas.
To put together the chimichangas, place about 3/4 to 1 cup of filling on a tortilla, fold the bottom over the filling, fold the sides in, then carefully fold/flip it over to close; repeat until you have as many chimichangas as you want to bake at one time.
Place the chimichangas on a cookie sheet, brush each one with 1/2 teaspoon of olive oil, and bake at 400 degrees F for 15 minutes.
Sprinkle each with grated cheddar, serve with extra sour cream, guacamole, and salsa, and enjoy!