In a medium mixing bowl, stir together the sour cream and mayonnaise until smooth and fully combined. This is your simple creamy sauce base.
Add the shredded chicken to the bowl and toss until every piece is well coated in the creamy mixture. The chicken should look evenly sauced and a bit thick.
Fold in 1 cup of the shredded cheddar cheese, reserving the remaining 1/2 cup for the top. Mix just until the cheese is evenly distributed throughout the chicken mixture.
Spoon the chicken mixture into the prepared glass baking dish and spread it out into an even layer, pressing it gently into the corners so it bakes evenly.
Sprinkle the remaining 1/2 cup shredded cheddar cheese evenly over the top, covering as much of the surface as possible so you get that bubbly, cheesy crust.
Place the casserole in the preheated oven and bake for 20–25 minutes, or until the edges are browned and crispy, the cheese on top is melted and bubbly, and the casserole is heated through.
If you want extra browning on top, switch the oven to broil for 1–2 minutes at the end, watching closely so the cheese doesn’t burn.
Remove the casserole from the oven and let it rest for about 5 minutes. This helps it set up slightly and makes it easier to scoop while still staying creamy and gooey inside.
Serve hot, scooping the cheesy chicken straight from the glass baking dish. Store any leftovers in an airtight container in the fridge for up to 3 days and reheat gently in the oven or microwave.
