1. Prepare the Filling:
   1. Finely chop the mushrooms (Optionally, reserve 3-4 large mushrooms to place in the center).
   2. Heat oil in a large frying pan. Add the onions and sauté until translucent, about 3 minutes. Add the mushrooms, season with salt and pepper, and fry until golden browned, about 5 minutes. Transfer onto paper towels to drain.
   3. Place the pan back on the heat with the rest of the oil. Sauté the garlic for a few seconds, then add the spinach and cook for 2-3 minutes until wilted. Transfer to a colander to drain and squeeze thoroughly (or simply thaw frozen spinach, and squeeze).
   4. Place the pan back on the heat & add ground tenderloin.  Brown, cooking completely through.  Drain extra fat.
   5. To a bowl, add cooked beef, rice, fried mushrooms and onions, sauteed garlic and spinach, chopped walnuts, mustard, fresh thyme, cheese, and oats (as needed to absorb the moisture and dry out the filling). Stir to combine, and adjust seasonings to taste. Wrap the mixture tightly in foil and refrigerate to cool completely.
2. Assemble the Loaf:
   1. Preheat the oven to 400°F (200°C).
   2. Put the puff pastry sheet on a baking tray lined with parchment paper. Place the filling in the middle of the pastry and unwrap. Press the large mushrooms (you've set aside) into the filling (optional).
   3. Carefully fold the sides of the pastry over the filling to create a well-packed loaf. Press the ends together to seal, then roll over the loaf so that the seams are on the bottom.
   4. Gently cut little slits on the top using a sharp knife (but do not cut all the way through). 
   5. Brush egg whites over the loaf.
   6. Bake the Wellington for approx. 20-25 minutes, until golden, puffed, and flaky.

Optional: Serve immediately with a creamy mushroom sauce or gravy