In a medium skillet, cook the bacon until barely crisp, about 5 minutes.
Drain on paper towels.
Discard all but 2 tablespoons of bacon fat from the skillet.
Add the butter; heat until melted; add the onion; saute over medium heat until golden.
Stir in the chicken livers; saute, stirring, just until cooked.
Add the tomatoes and sage; cook, stirring occasionally, until sauce thickens slightly, about 10 minutes.
Season with salt; add pepper to taste.
Cut bacon into 1/4-inch pieces and stir into sauce.
Meanwhile, cook the fettuccine in plenty of boiling salted water until al dente, or firm to the bite, 2 minutes for fresh and 5 to 7 minutes for dried; drain.