Start a day ahead. Mix the olive oil, ginger, garlic, sherry and soy sauce together. Place the fillet in a shallow dish. Pour the marinade over the meat. Cover and place in the fridge overnight, turning occasionally.
Set oven to Gas Mark 6, 200OC (400OF). Lift the meat onto a roasting tin. Reserve the marinade. Roast the fillet, allowing 8 minutes per pound for rare and 10 minutes per pound for medium, or use a meat thermometer. Brush the fillet during cooking with the marinade.
When the meat is cooked, set aside until you are ready to assemble. The beef is best cooked on the day and still slightly warm. 30-40 minutes before serving place the beef on the bread and top with herb dressing.
Have all the ingredients at room temperature. Combine the egg, mustard and salt in the food processor. Then slowly pour in the oil. When the dressing is quite thick add the garlic, soured cream and herbs.