Heat the oven to 375 degrees F. Season the chicken breasts with salt and pepper then set in a pan and bake for about 20-25 minutes until they are 165 degrees F. when tested with an instant-read meat thermometer. Remove from the oven and cool enough to shred the meat with a couple of forks. Alternatively, you can use leftover chicken or even a rotisserie chicken.
In a large skillet over medium-high heat, saute the chopped onions in two tablespoons of oil for 2-3 minutes over medium-high heat just until they start to soften. Add the diced green chilies and cook for another minute.
Sprinkle the flour, cumin, salt, and garlic powder over the onions and green chilies and stir, cooking for about 30 seconds before adding the green chili salsa or salsa verde, shredded chicken, and chicken broth, then stirring to combine.
Cook for about 5 minutes until the mixture has thickened and the shredded chicken has absorbed the juices, then remove from the heat.
To make each chimichanga, warm the tortillas so they are pliable, then scoop a generous amount (about ½ cup) of the green chili chicken filling onto each tortilla. Fold in both sides and the bottom of each tortilla over the chicken filling, then rolling it up just like you would a burrito, tucking in the sides to create a tight packet. Secure with toothpicks.
Heat a large skillet with enough oil to cover the bottom of the pan over medium-high heat. You can deep fry the chimichangas if you want, but I find that about 1 cup of oil is more than enough to pan-fry all of my chimichangas this way. Working in batches, carefully place 3-4 chimichangas, seam side down, into the hot oil and fry until golden brown and crispy. Use tongs to carefully flip each chimichanga and fry on the other side until crispy and brown as well.
Remove the cooked chimichangas from the pan and set on a paper towel-lined plate until all chimichangas are cooked. Remove the toothpicks, then serve the warm chimichangas with all of the fixings.
Leftovers can be stored in the fridge, although they won't be as good the next day.