Put flour in saucepan, gradually add milk, whisking until smooth. Cook over low heat,stiring til thickened (about 5 min). Add butter, stir til melted, stir in half&half and heavy cream stir til well combined. Add grated cheese and stir until melted, removefrom heat and stir in salt and pepper.
Put the sauce in the bowl with the fettuccine, and toss until pasta is well coated. Serve immediately.