Preheat your oven to 500°F and lightly grease a 9x13 inch baking dish or similar size casserole. While the oven heats, cut the chicken into 1-inch bite-sized cubes and set aside. Finely grate the Monterey Jack cheese and parmesan cheese (freshly grated cheese melts much more evenly than pre-shredded), and measure out all remaining ingredients so everything is ready to assemble quickly.

In a medium bowl, whisk together the mayonnaise, sour cream, garlic salt, onion powder, and pepper until smooth and well combined. The sauce should be thick and creamy. I like to taste a small spoonful at this point to adjust the seasoning if needed—this is your opportunity to balance the flavors before everything bakes together.

Spread the spinach evenly across the bottom of the prepared baking dish, breaking up any clumps. Distribute the chicken cubes evenly over the spinach layer. Pour the creamy sauce from Step 2 over the chicken, using a spatula to gently spread it and ensure everything is coated. Finally, sprinkle the grated Monterey Jack and parmesan cheeses evenly over the top.

Place the casserole on the bottom rack of your preheated 500°F oven and bake for 20–25 minutes, until the cheese is melted and golden brown and the sauce is bubbling around the edges. The chicken will cook through quickly at this high temperature, and the bottom rack helps ensure the spinach softens while the top gets nicely browned. Let the casserole rest for 2–3 minutes out of the oven before serving—this helps the sauce set slightly and makes it easier to portion.