Heat the butter and oil in a large frying pan and sauté the onion until translucent. Add the flour and continue to cook while stirring for a further minute. Pour in the chicken stock and continue to stir until the sauce has thickened slightly.
Add the mango and chicken and simmer for 20 minutes. Stir in the fresh cream and season to taste. Continue cooking until the sauce is thick and creamy for about 2 minutes.
In plenty of salted boiling water, cook the fettucine until al dente and drain.
Toss the sauce and chicken through the pasta, add the walnuts, and serve topped with the fresh basil.