Line a 9-by-13-inch baking dish with 2 pieces of foil.
Put the chicken in a large pan, season with salt and pepper and cover with water. Bring to a boil, lower to a simmer and simmer until the chicken is cooked through, about 45 minutes. Remove the chicken and let cool, then shred the meat, discarding the skin and bones. Save the broth for cooking the spaghetti.
Remove the sausage from the casings and put into a large skillet with the olive oil. Cook over medium heat, breaking up the sausage meat into small pieces, until it is no longer pink, 5 to 6 minutes. Remove and drain on paper towels. Add the mushrooms to the skillet. When they give up their liquid, add the green peppers and sundried tomatoes, then cook until just beginning to soften, 5 to 7 minutes. Remove and set aside on top of the sausage. Reserve the oil in the skillet.
Bring the reserved chicken broth back to a boil. Cook the spaghetti in the broth until al dente according to the package instructions, then drain, reserving the broth.
If there is less than 2 tablespoons oil/fat in the skillet, add some olive oil to make 2 tablespoons. Whisk in the flour and cook for 1 minute over medium heat. Whisk in 1 1/2 cups of the reserved chicken broth. When it starts to thicken, after about 5 minutes, add the half-and-half. If it is too thick, thin with some more broth. Turn off the heat and add the basil and 1/2 cup of the Parmesan; season with salt and pepper to taste. Stir in the chicken, sausage and vegetables. Stir in the spaghetti.
Pour the mixture into the prepared baking dish, sprinkle the remaining 1/4 cup Parmesan on top and cover with aluminum foil. Cook the casserole immediately, or let cool completely, then freeze.
If cooking right away, preheat the oven to 375 degrees F. Cook for 25 minutes, then uncover the baking dish and cook until heated through, an additional 20 minutes.
To cook from frozen: Preheat the oven to 325 degrees F.
Cook the frozen casserole for 2 hours, then raise the temperature to 375 degrees F, uncover and cook until heated through, 15 minutes more.