Heat half the oil and half the butter in a saucepan over a low heat, add the onions and gently cook for 10 minutes until soft and just starting to turn golden.
Meanwhile, rinse the liver, then pat dry with paper towels. If it is not already sliced, cut into thin slices. Toss each piece in flour that has been seasoned with salt and pepper.
Add the tomatoes, stock, Worcestershire sauce, tomato paste and herbs to the onions and bring to the boil. Reduce the heat, cover and cook for 8–10 minutes until thickened, stirring occasionally.
Meanwhile, heat the remaining oil and butter in a large frying pan over a moderate heat until sizzling. Add the liver and bacon and fry for 4–6 minutes, turning, until the liver is just cooked and the bacon is lightly browned and crispy.
Remove the liver and bacon from the pan using a slotted spoon and put onto warm serving plates. Spoon the onion and tomato sauce on the side, then sprinkle with parsley. Serve immediately with sour cream.