Roast the yellow pepper in a hot oven or under a preheated hot grill, skin-side up, until the skin is blistered and black. Place in a plastic bag and allow to cool, then peel off the skin. Cut the flesh into strips.
Cook the penne in lightly salted boiling water, according to the packet instructions.
Meanwhile, heat the oil and butter in a large frying pan, add the sliced onion and chicken livers and cook over a high heat until browned all over. Add the rosemary, the strips of yellow pepper and seasoning. Do not overcook the livers as they will become dry and hard – they are best when still pink in the middle.
Mix the chicken liver mixture with the cooked pasta and toss well. Serve immediately with the Parmesan.