| STUFFING: Puree the mushrooms and onions together in a food processor. Melt the butter in a skillet over medium heat, add the mushroom and onion mixture and cook, stirring occasionally, until the mixture is dry. Add salt and pepper to taste and mash in the pate. Transfer the stuffing to a bowl and chill in the refrigerator.
MEAT: Heat the oil in a skillet over high heat. Add the beef and sear on all sides. Remove the beef from the skillet and sprinkle with salt and pepper. Place in the refrigerator until cold.
ASSEMBLE THE WELLINGTON: Roll out chilled puff pastry into a rectangle large enough to enclose the beef. Spread the dough with a generous layer of stuffing, place the beef on top and spoon another bit of stuffing on the beef. Wrap the pastry to enclose the beef and stuffing and place the Wellington in the refrigerator to chill for at least 15 minutes.
COOK THE WELLINGTON: Preheat oven to 425F. Place Wellington in oven and immediately lower heat to 350F. Roast for about 20 minutes.