Cook spaghetti according to package directions - but only cook to half the al dente time. (If the package says to cook for 6 minutes for al dente, only cook your pasta for 3 minutes.) Drain pasta and place in large bowl.
In a medium bowl, combine soup, sour cream, milk, salt, and pepper. Fold in turkey and mushrooms.
Add turkey mixture to spaghetti, toss to mix.
Pour spaghetti and turkey mixture into a lightly greased casserole slow cooker. Top with cheese.
Cover and cook on low for 3-4 hours, until hot.