Preheat the Oven
Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.
Prepare the Filling
In a medium bowl, combine shredded chicken, diced onions, cheddar cheese, garlic powder, chili powder, cumin, salt, and pepper. Mix until evenly combined.
Make the Sour Cream White Sauce
Melt butter in a medium saucepan over medium heat.
Stir in flour and cook 1–2 minutes to remove raw flour taste.
Gradually whisk in milk, ensuring no lumps. Cook 3–5 minutes until thickened.
Remove from heat and stir in sour cream, garlic powder, onion powder, salt, and pepper. Sauce should be smooth and creamy.
Assemble the Enchiladas
Spread a small amount of sauce on the bottom of the prepared baking dish.
Place about ¼ cup of chicken filling in the center of each tortilla.
Roll tortillas around filling and place seam-side down in the baking dish. Repeat with remaining tortillas.
Add Sauce and Cheese
Pour remaining sour cream sauce evenly over the rolled tortillas. Sprinkle shredded Monterey Jack or Mexican blend cheese on top.
Bake
Cover with foil and bake for 20 minutes. Remove foil and bake an additional 10 minutes until cheese is melted and slightly golden.
Garnish and Serve
Let enchiladas rest 5 minutes. Garnish with chopped cilantro and sliced jalapeños if desired. Serve hot.