Heat the oil in a nonstick frying pan and cook the bacon for 3–4 minutes, until crisp and golden. Remove from the pan with a slotted spoon and set aside. Season the chicken livers with salt and pepper, and add to the hot pan. Cook for 4–5 minutes, until browned and cooked through.
Combine all the dressing ingredients in a jar with a tight-fitting lid and shake well.
Arrange the tomatoes on 4 plates with the watercress and red onion. Scatter the liver and bacon over the prepared salads and serve immediately, drizzled with the dressing.