Preheat oven to 350 degrees. Spray a 9X13 inch baking dish with nonstick cooking spray; set aside.
Cook spaghetti according to package instructions, in unsalted water, just until al dente. Drain. Return pasta to the cooking pot.
While spaghetti cooks, make the sauce. In a large mixing bowl stir together soup, sour cream, milk, Ranch dressing mix, dill, chicken, half of the cooked bacon, and 1 cup of cheddar cheese until mixture is well combined and smooth.
Pour sauce over pasta, toss to evenly coat. Spread coated pasta evenly in prepared baking dish. Spray a sheet of aluminum foil, large enough to cover baking dish, with nonstick cooking spray. Cover dish, cooking spray side down, and bake in preheated oven for 40 minutes.
Remove from oven, uncover, sprinkle top evenly with remaining 2 cups of cheddar and remaining bacon. Return to oven, continue to cook until cheese melts and casserole is heated through, about 10-15 minutes.
Garnish with parsley and serve.