Heat the butter and oil in a frying pan, add the onion and cook for 5 minutes until softened and lightly browned. Add the kidneys and fry over a high heat for 3 minutes until browned.
Add the tomatoes and cook for 2 minutes, then stir in the mustard, tomato purée, Marsala and salt and pepper. Cook for 2–3 minutes, stirring, until the sauce has reduced slightly and the kidneys are cooked. Cover with a lid and keep hot.
Wind the bacon around eight metal skewers and grill for 8–10 minutes until crisp. Toss the rocket leaves in the vinegar. Toast the bread and cut each slice in half.
Arrange the toast on serving plates, reheat the kidneys if necessary and spoon them on to the toast. Slide the skewers from the bacon and arrange the bacon attractively on the kidneys. Spoon the rocket salad on the side and serve immediately.