Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Season the beef with salt and black pepper.
In a large skillet, heat half the cooking oil. So as not to crowd the pan, you might have to cook the beef in two batches. Brown the pieces of beef quickly on all sides, only taking about 2 minutes. Remove to a plate lined with a paper towel.
In the same skillet, on medium-low heat, add the remaining oil, then add the onion and mushrooms. Sauté for 10 minutes, stirring occasionally. Add soy sauce, oregano, black pepper and stir well.
Transfer the mixture to a food processor, along with the pitted olives. Process to a chunky mixture.
Unroll the puff pastry sheet and cut it into 3” squares. Spread a little mustard in the center of each square, then spread on some mushroom mixture. Top with a piece of beef.
Fold two opposite tips of the pastry over the beef and press them together. Fold the other two over the top and press.
Set the wellingtons on the baking sheet. When they are all done, brush them with beaten egg.
Bake for 10-15 minutes or until the pastry is puffed and golden brown.
To make the sauce, add the onion to the skillet once the mushroom mixture has been removed. Saute for 5 minutes on medium-low. Add the garlic and saute for 30 seconds more.
Turn the heat up to medium. Pour in the white wine and whisk. Cook for 2-3 minutes, until the wine is reduced by half.
Pour in the beef stock, heavy cream, and Dijon. Whisk to combine. Cook at a low boil for about 5 minutes, until the sauce has reduced and thickened, slightly. Add to a food processor or blender and blend until smooth.
Serve with the mini beef wellingtons.