In large saucepan, cook pasta to desired doneness as directed on package. Drain in colander or strainer; cover to keep warm.
In same saucepan, melt margarine over medium-high heat. Add milk and sauce mix; stir with wire whisk until well blended. Bring to a boil, stirring constantly. Reduce heat. Add spinach; simmer 3 minutes, stirring occasionally.
Add cooked pasta, tuna, mushrooms and tomatoes; stir gently to coat. Cook 3 to 5 minutes or until thoroughly heated.