I used to cook this in a west London restaurant. My approach would be to add a little more of everything than was needed for the portion. Then after the order was plated, I would eat the rest from the hot pan. This tended to infuriate the head chef, Toby, seeing me with gravy on my chin taking hurried mouthfuls as he called the next order. A good little meal when alone.
Bring a small saucepan of water to the boil. Add the peas and cook for 2 minutes. Drain in a sieve, then set aside. Trim the chicken livers, cutting out any sinew and unsightly bits. As the rest of this recipe is so quick to cook, have all remaining ingredients that you will need standing to attention â€“ a bit like cooking in a restaurant kitchen. If you donâ€™t, the chicken livers will overcook as you faff around; they should be just done, cooked to pink. Cut the bacon into thin strips (I like to buy bacon in 1 piece and cut little matchsticks, but you can also use rashers). Peel the garlic and chop finely. Strip the thyme leaves from the sprig and finely chop. Roughly tear the mint leaves.
Heat the oil in a small frying pan over a medium-high heat and fry the bacon for 3-4 minutes until beginning to colour. At this point, season the chicken livers with salt, ground black pepper and the thyme. Add them to the pan and fry until well coloured on one side â€“ this should take around 30 seconds, providing the livers are sizzling the minute they hit the pan.
Flip the livers over and add the garlic. Cook for a few seconds so they colour on the other side, then shake the pan and toss everything around a little. Pour about the sherry vinegar to deglaze the pan and cook until it has almost evaporated. Add the peas, followed by the stock. Cook until the liquid has reduced by half. Blob, then swirl in the mustard. Add the cold cubed butter and swirl the pan around until a glossy thickened sauce has formed. Spoon on to a soup plate, scatter with the torn mint and eat at once. Make sure that you have some good bread to chase the last of the tasty gravy around, or by all means lick the plate.