Heat a large saucepan or deep frying pan. Melt half of the butter, add the olive oil; once heated add the finely chopped onion.
Cook for approximately 3 minutes, until the onion is soft (do not brown) then add the seasoned chicken livers and cook slowly on a low heat for 20 minutes, stirring occasionally. With a slotted spoon, remove the chicken livers and finely chop them then add back to the pan, stir and season.
To a large pot of boiling salted water, add the tagliatelle and cook until al dente; drain into a colander and add to the sauce along with the remaining butter. Toss well for a minute or so to ensure the pasta absorbs the sauce and serve immediately with a generous helping of black pepper.
Wine paring: Rosato di Bolgheri