Heat the skillet over medium heat. Add a little oil if desired, then sauté the diced onion until it is translucent, about 5 minutes.
Add the minced garlic, cumin, chili powder, salt, and pepper to the skillet. Cook for another 1-2 minutes until fragrant.
Stir in the shredded chicken, cooked rice, black beans, and corn. Mix well and heat through for 5 minutes.
Remove the skillet from heat and stir in half of the shredded cheese, and the salsa.
Warm the flour tortillas in a separate pan or microwave for about 30 seconds to make them pliable.
Divide the chicken mixture evenly among the tortillas, placing the filling in the center of each.
Sprinkle the remaining cheese over the filling, and add fresh cilantro if using.
Fold the sides of each tortilla in and then roll it away from you tightly to form a burrito.
Optionally, wrap the burritos in aluminum foil or parchment paper to hold them together.
Serve warm with sour cream on the side if desired.