Meanwhile, over a moderate heat in a large pan that can accommodate the cooked pasta, heat the olive oil and butter. Saute the onions until golden brown, about 5 minutes. Add the vodka, stir, and when most of the liquid has evaporated, add the cream and stir for about 5 minutes. Season with black pepper, chili flakes, if using, lemon rind and chives, stirring for about 1 minute. Add the smoked salmon and stir carefully until well combined.