Preheat the oven to 425°F (220°C). Place the beef tenderloin in a baking dish and spread 2 tablespoons of softened butter evenly over the surface.
Roast the tenderloin for 10–15 minutes, until lightly browned. Remove the beef from the baking dish, reserving the pan juices for the sauce. Transfer the beef to a plate and allow it to cool completely.
Increase the oven temperature to 450°F (230°C).
Melt 2 tablespoons of butter in a skillet over medium heat. Add the chopped onion and sliced mushrooms. Sauté for about 5 minutes, or until softened and most of the excess moisture has cooked away. Remove from the heat and cool.
In a small bowl, combine the halal pâté with the remaining 2 tablespoons of softened butter. Season lightly with salt and black pepper.
Spread the pâté mixture evenly over the cooled tenderloin. Spoon the mushroom and onion mixture over the beef, pressing it gently into place.
Roll the thawed puff pastry to approximately ¼-inch thickness. Place the prepared tenderloin in the center.
Fold the pastry around the beef and seal the edges securely. Avoid creating excessively thick pastry seams. Place the wrapped tenderloin seam-side down in a baking dish or on a lined baking sheet.
Cut a few small slits into the top of the pastry to allow steam to escape. Brush the surface evenly with beaten egg yolk.
Bake for 10 minutes at 450°F (230°C). Reduce the oven temperature to 425°F (220°C) and continue baking for approximately 10–15 minutes, or until the pastry is deeply golden and crisp. For medium-rare beef, check the center with an instant-read thermometer and account for a small temperature rise while resting.
Remove the Wellington from the oven and allow it to rest before slicing.
Meanwhile, prepare the alcohol-free sauce. Pour the reserved beef juices into a small saucepan and add the halal beef broth, red grape juice, and balsamic vinegar.
Bring the sauce to a boil, then simmer until slightly reduced, about 5 minutes. Strain if desired.
Slice your Beef Wellington Recipe carefully with a sharp knife and serve warm with the reduced sauce.