Set your oven to 250°F (120°C).
Place the wellington (sliced or whole) on a wire rack.
Cover the wellington loosely with foil, creating a few slits in the foil to act as vents.
Put the wellington in the oven, and place a baking tray beneath the wire rack to catch any drippings.
Heat beef wellington slices for 15-25 minutes. Whole beef wellingtons will need between 40-60 minutes depending on size.
Check the temperature of the meat* (ideally using a meat thermometer) before removing the wellington from the oven. This is especially important for whole beef wellingtons.
To crisp the pastry, take the foil off for the last 4-5 minutes of heating.
Let the beef wellington rest for a few mintues before serving.