Place the beef in a large bowl and add all the marinade ingredients. Cover and marinate overnight and keep refrigerated.
Drain off the liquid from the marinade and reserve. Remove the peppercorns, onions, and garlic. Separate the vegetables from the meat. Dry the meat thoroughly on paper towels.
Heat a heavy bottomed saucepan. Add the butter and oil, then add in the beef. Cook over a high heat until seared all over, then remove from the pan and keep warm. Add the bacon and cook for 3 to 4 minutes, until golden. Add the shallots and cook until they just begin to soften. Stir in all the marinade vegetables, and cook for a further 3 to 4 minutes. Return the meat to the pan and season generously with salt and pepper and stir in the mustard until all the ingredients are coated generously. Pour in the reserved marinade liquid and bring to a gentle bubble. Cover with a lid and cook for 1 1/2 to 2 hours, the sauce should be thick and the meat tender. (If the sauce is not thick, take 1/4 cup of the liquid and with an whisk, mix in 2 tablespoons of flour until smooth. Mix this back into the pot with the other liquid until it is absorbed, and cook for another 15 or so minutes until it thickens.) If there is not enough liquid to cover the beef and vegetables, add some beef broth half way through the cooking process. When you have reached the desired consistency, serve individually in large bowls (such as a pasta bowl) with the mashed potatoes.
Cook the potatoes in salted boiling water for 12 to 15 minutes until tender. Heat the cream and butter together until the butter is thoroughly melted. Place the herbs in a blender and slowly add the olive oil, while the motor is still running, to form a smooth puree.
Mash the potatoes, then add butter and cream. Stir thoroughly until you have a smooth paste. Finally fold in herb puree and season with salt and pepper. Mix well to form a green smooth mash.