For the beef pieces:

• 1 pound beef tenderloin (450 grams): The star of the dish, providing rich, tender bites.
• ½ teaspoon salt: Enhances the beef’s natural flavors.
• ¼ teaspoon black pepper: Adds a subtle heat and depth.
• 1 tablespoon olive oil: For searing the beef, adding a hint of richness.

For the mushroom filling:

• 1 tablespoon olive oil: Used to sauté the onions and mushrooms.
• ½ onion, minced: Offers a sweet, aromatic base to the filling.
• 2 cups finely chopped button mushrooms (about 7 ounces/200 grams): The earthy component of the filling.
• 1 tablespoon regular soy sauce: Brings umami and depth to the mixture.
• A generous pinch of black pepper: Complements the mushrooms’ earthiness.
• 1 teaspoon dried oregano: Adds a hint of herbaceous flavor.
• ½ cup pitted black olives (60 grams): Introduces a briny contrast.

For beef Wellington bites:

• 1 pound puff pastry (450 grams): The flaky, buttery envelope that encases the filling.
• Mustard, to taste: Adds a tangy, sharp accent.
• 1 small egg, beaten: Gives the pastry a golden sheen.