Preheat the oven to 375°F. Add 2 tablespoons of the butter to a large pot over medium heat. Once melted, add the onion and garlic and sauté until translucent.
While you're sautéing the garlic and onion, cook your spaghetti noodles al dente (to do this, subtract 3 minutes from the package instructions).
Add the chicken broth, milk, cream cheese, Rotel, half of the shredded Monterey jack cheese, half of the shredded cheddar cheese, and 1 teaspoon of salt to the pot with the garlic and onion. Stir until everything is melted.
Sprinkle the flour on top of the melted cheese sauce and whisk until smooth. Let the sauce simmer for 5 minutes, stirring and scraping the bottom every minute or so.
Once the sauce has thickened, stir in the chicken.
Toss the bread crumbs with ¼ cup of melted butter and a ½ teaspoon of the salt.
Place the al dente noodles in the bottom of a 9x13 casserole dish, pour the chicken and cheese sauce over top and toss to combine. Spread the cheesy noodles out in an even layer, top with an even layer of the rest of the shredded cheese, and then an even layer of the buttery bread crumbs.
Bake at 375°F for 25 minutes and then broil for 3-4 minutes, until the bread crumbs are golden brown. Garnish with parsley, serve, and enjoy!