- Attach the sous-vide precision cooker to the side of a cooking container. Add enough water to reach the midpoint between the minimum and maximum required. Set the temperature to 145°F (63°C). Set the cooking time to 3 hours. Preheat the water while you prepare the turkey.
- On a work surface, sprinkle the salt over the entire surface of the roast. Season with pepper. Place the roast in a sous vide bag or a freezer bag with a slider closing. Add the butter and whole-grain mustard. Remove all the air from the bag. Submerge the bag in the preheated water. Begin cooking.
- Once cooked, remove the bag from the water. At this point, the roast can be refrigerated, if desired. Remove the roast from the bag and place on a plate. Transfer the butter mixture from the bag into a bowl.
- In a large skillet over high heat, brown the roast on all sides in 3 tbsp (45 ml) of the reserved butter mixture. Set the roast aside on a plate and tent with foil.
- In a bowl, whisk together the broth, cream, Dijon mustard and toasted flour. Set aside.
- In the same skillet over high heat, brown the mushrooms in 2 tbsp (30 ml) of the reserved butter mixture. Season with salt and pepper. Set aside on another plate.
- Pour the broth mixture into the skillet. Bring to a boil while whisking constantly. Simmer over high heat for 5 minutes while stirring. Return the mushrooms to the skillet. Adjust the seasoning.
- Meanwhile, in a small skillet, brown the chorizo in 1 tbsp (15 ml) of the reserved butter mixture. Add the chives and cook for 1 minute.
- Slice the roast and arrange on a serving dish. Cover with the sauce. Top with the chorizo mixture and the chervil. Serve with vegetables and mashed potatoes, if desired.