Preheat your oven to 375°F. Lightly grease a 9x13-inch baking pan and set it aside.
In a large pot of boiling lightly salted water, cook the spaghetti until it reaches an al dente texture. Drain the noodles and toss them gently to prevent sticking while cooling.
Meanwhile, blend together the milk, chicken broth, and flour until completely smooth. Set the mixture aside.
Place the drained pot over medium heat. Melt the butter and stir in the onion powder, garlic powder, basil, oregano, salt, pepper, and thyme. Sauté the spices for about 30-45 seconds until fragrant and sizzling.
Pour in the milk blend while whisking continuously. Bring the sauce to a gentle simmer and cook for 3-4 minutes until it thickens slightly.
Remove the pot from the heat. Stir in ½ cup of the Parmesan cheese and 2 cups of the cheddar cheese until smooth and velvety. Taste and adjust the seasoning if needed.
In a large bowl (or return the sauce to the pot if preferred), combine the cooked spaghetti and chopped chicken with the cheesy sauce, ensuring every strand is well coated.
Transfer the sauced pasta and chicken mixture into the prepared baking pan. Sprinkle the remaining Parmesan and cheddar cheeses generously on top.
Bake for 20-25 minutes until the casserole is bubbling along the edges and the top is lightly golden.
Remove from the oven, garnish with fresh chopped parsley (if using), and serve immediately while the sauce is delectably creamy.
