Beef Wellington Bites

Ingredients

• 1 pound beef tenderloin, cut into 1-inch pieces
• 1/2 teaspoon salt
• 1/4 teaspoon ground black pepper (or to taste)
• 1 tablespoon olive oil (for browning beef)
• 1/2 onion finely chopped
• 2 cups button mushrooms, very finely chopped ((about 7 ounces))
• 1 tablespoon regular soy sauce
• 1 teaspoon dried oregano
• generous pinch black pepper
• 1/2 cup pitted black olives
• 1 pound puff pastry sheet (thawed and unrolled)
• as much as you like mustard (for spreading)
• 1 small egg, beaten (for egg wash)