Cook spaghetti according to package directions, adding bouillon and onion to cooking water. Drain and place in a large bowl.
Stir together soup, sour cream, milk, salt and pepper in a medium bowl. Fold in turkey, mushrooms and peas, if using. Lightly stir mixture into spaghetti, coating well. Pour into a lightly greased 13"x9" baking pan; top with cheese.
Bake, covered, at 350 degrees for 30 to 40 minutes, until hot and bubbly.