Preheat your oven to a cozy 350°F (175°C), setting the stage for a culinary delight. In a large pot of boiling water, introduce the chicken pieces and let them simmer into tenderness—this is where our casserole's flavor foundation starts.
Once the chicken is tender, reserve 2 cups of broth, then shred the chicken after it's cooled. Plunge the spaghetti into the same flavorful broth, cooking it just to al dente.
In a gathering bowl, unite the shredded chicken, al dente spaghetti, creamy mushroom soup, and a bounty of cheddar cheese with the bell peppers, onion, and spices. Add in just enough reserved broth to bring everything together in a moist, but not wet, embrace.
Lay the mixture in a casserole dish, lovingly top with the rest of the cheddar, and choose your next move—to bake now, refrigerate for later, or freeze for a future feast.
If baking immediately, watch the cheese bubble and turn golden for about 45 minutes—cover with foil if it's browning too fast, like a quick cloud cover on a sunny day.