Preheat oven to 425°F. Grease a 2-quart (about 11 x 7-inch or 9-inch square) baking dish or spray with nonstick cooking spray. Set aside.
In a large bowl, whisk together chicken broth and condensed soup until smooth. Add the uncooked spaghetti, ham, mushrooms, garlic, and ground black pepper. Stir to combine everything together.
Transfer the mixture to the prepared baking dish, using your hands or a wooden spoon to gently press all of the ingredients into an even layer. Cover tightly with aluminum foil and bake for 45 minutes.
Uncover; stir gently. At this point, check the pasta to make sure that it is al dente (firm but just about finished cooking). If it’s still too hard, cover the dish and return to the oven until pasta is al dente. Then, move on to the next step.
Stir in the frozen peas and sprinkle the shredded cheese on top. Return the dish to the oven (uncovered), and bake until the pasta is tender and the cheese is melted, about 5-10 more minutes.
Garnish with chopped fresh parsley and serve.
Notes
Total cooking time will vary depending on the type of pan that you use and on your individual oven. A glass or ceramic casserole dish will take longer than a metal pan, for instance. The metal pan may only need 40 minutes before the pasta is al dente. To know when your casserole is done, taste a bite of the pasta. The noodles should be tender and the sauce should be thicker.
Be careful not to overbake the casserole or the pasta will become mushy.
The sauce will continue to thicken as the casserole rests and cools.
Use low-sodium chicken broth or water. The soup and ham add a good amount of salt on their own.
Try seasoning the dish with other herbs and spices that you enjoy. Good options include garlic powder, onion powder, paprika, thyme, rosemary, basil, oregano, chives, and parsley.
Cooking for a smaller family? Cut all of the ingredients in half and bake the casserole in an 8-inch square dish. The cooking instructions remain the same.