Cook pasta according to package directions; drain.
Meanwhile, for sauce, in a medium saucepan cook the green onion in margarine or butter over medium-high heat for 1 minute. Reduce heat. Add the mascarpone or cream cheese. Cook and stir until cheese melts. Add milk and stir until sauce is smooth. Stir in Parmesan cheese.
Pour sauce over pasta; toss to coat. Sprinkle with toasted walnuts. Makes 4 side-dish servings.