Cook fettuccine according to package directions; drain. Add oil; toss to coat.
In a large saucepan, combine flour and milk until smooth. Stir in the onions, basil, salt, garlic powder, lemon zest and pepper. Bring to a boil; cook and stir for 2 minutes or until slightly thickened. Remove from the heat; stir in cheese until blended. Pour over fettuccine and toss to coat. Sprinkle with additional cheese and basil if desired.