In 8-quart pan, boil chicken, 1 onion, pepper, and 1 tablespoon salt. Reduce heat to low and simmer until chicken is fork-tender (about 30 minutes). Remove chicken.
Strain the broth. Discard the solids.
When chicken has cooled, cut into 2 cm cubes.
Cook spaghetti according to package directions. Strain and spread out in greased 13" by 9" casserole.
Chop the other onion. In 4-quart pot over medium heat, melt 2 tablespoons butter. Cook onion, mushrooms, and lemon juice for 5 minutes. Remove using a slotted spoon.
Melt remaining butter. Add flour, 2 teaspoons salt, nutmeg, and 1/2 teaspoon paprika. Stir until smooth.
Gradually stir in sherry and 3.5 cups of chicken broth. Cook and stir until liquid is thickened.
Add half-and-half, chicken, and mushroom mixture. Cook over low heat until warm.
Spoon chicken mixture over spaghetti. Top with grated Parmesan cheese and paprika.
Bake in oven at 350 F for 30 minutes. The surface should be golden and bubbly.