Preheat oven to 375 degrees Fahrenheit.

Grease a 9x13-inch baking dish with butter or a drizzle of oil and set aside.

Add chicken thighs or breasts to a medium-large pot and cover entirely with water.

Bring to a boil over medium-high heat. Once boiling, reduce heat to low and cover with a lid.

Depending on the size, boil the chicken for 15-25 minutes. Check the temperature with a meat thermometer. When inserted in the thickest part of the meat, it should read 165 degrees Fahrenheit.

Remove chicken from the liquid, transfer to a large bowl, and shred using two forks.

In the meantime, add spaghetti to a pot with water; if needed, break it in half. Add a drizzle of oil and stir. Cook per package instructions, aiming for al dente or 1-2 minutes shy of the package cooking time.

Once pasta is cooked, reserve 2 cups of cooking liquid and discard the rest. Drain the pasta.

Add cooked spaghetti, condensed cream of mushroom soup, sour cream, grated cheese, diced onion, garlic, sliced mushrooms, Ranch seasoning, and half of the bacon to the large bowl with shredded chicken. Stir to combine.

Add 1 cup of broth or reserved cooking liquid and adjust for salt and pepper. If the mixture needs more liquid, add 1 cup and stir well to combine.

Transfer the mixture to the prepared baking dish and distribute it evenly.

Combine cracker crumbs with melted butter in a medium bowl until well blended.

Spread the crumb mixture on top of the casserole. Sprinkle the remaining 1 ½ cups of cheese on top. Distribute evenly.

Add the remaining bacon.

Bake for 30-40 minutes or until the mixture is bubbly and the cheese fully melts.