Prepare the veal as necessary: cut or truss if needed (see recipe post for details) and then season with the salt and pepper.
Cut the carrots, celery and onions. Put them in the roasting pan, season and drizzle in the oil then add the fresh thyme. Place the veal on top (or on a rack if your roasting pan has one).
Roast the veal and vegetables at 65C / 325F for 50-60 minutes or until an internal read thermometer reads at least 49C / 120F.
Remove from oven and allow to cool before the assembly or see notes on making ahead and storing.
Onion Soubise
In a medium size pot bring water to boil (about 1.5 litres or 6 cups).
When the water is boiling, add the rice to the pot and cook for 5 minutes then drain and put it in a medium-sized oven-safe dish.
Slice the onions and add them to the oven safe dish on top of the rice along with the water, salt and half the melted butter.
Cover the dish and put in the oven (at the same time as the veal if possible) and cook for 17-18 minutes. The onions will be soft and translucent and the rice cooked through. Allow to cool a few minutes.
Blend, mix or mash the onion soubise mixture to as fine a consistency as you can get. You can use a food processor (ideally), blender, or any other mixer like a mortar and pestle if that's all you have. It should be thicker than a cream soup but not as thick as mashed potatoes. If it seems too thick or clumps together, add 1 teaspoon of water at a time until it reaches the consistency of yogurt.
Once blended, set aside until assembly.
Mushrooms Duxelles
Wash and finely chop the mushrooms. Finely chop the shallots.
Melt half the butter in a non-stick skillet over medium-high heat (bit don't brown it).
Add the mushrooms and shallots to the skillet and season with the salt and black pepper.
Cook 3-5 minutes or more, until the mushrooms have softened and the liquid that comes out of them has evaporated.
Once the water is evaporated, add the remaining butter and cook another 2-3 minutes until aromatic and (ideally) beginning to brown. If the mushrooms are cut very fine like through a food processor, then they likely won't brown.
Once cooked, set aside until assembly.
Mornay Sauce
Measure out the butter, flour and milk (and pan roasting juice if using) and set aside close to your cooktop.
Heat a heavy-bottomed sauce pan over medium-high heat.
Add the flour and butter to the pan and cook 3-5 minutes or until it smells nutty.
Add the milk a little at a time while whisking vigorously. The key to avoiding lumps in bechamel is to whisk firmly and continually. Repeat until all milk has been added and the sauce is smooth and creamy. For troubleshooting see recipe post.
Season with salt, black pepper and nutmeg. Then add the cheese and mix until it's melted in.
Assembly
When ready to cook, preheat oven to 200 C / 400 F.
Slice the cooled veal roast into approximately 1.5 cm / 0.5 inch slices (or slightly thicker is fine).
Starting with an end piece, smear it with the onion soubise, and then spoon on some mushroom duxelles. The mushrooms should easily stick to the onion soubise. Place the veal in the roasting pan and repeat with each slice, arranging them in their original shape but now with sauce between the layers.
Once the sauces have been added to all the layers and the layers are arranged in the pan, add the roasting vegetables (if made) and boiled potatoes* (if made) to the pan.
Cover everything with the mornay sauce. You can optionally sprinkle some more grated Gruyere cheese over the top.
Bake uncovered for 15-18 minutes or until the white sauce is golden brown.
Notes
* I added boiled potatoes to the dish but they are not included in this recipe as they are optional. As such, the nutritional information is only for the Veal Orloff recipe not for any side dishes.