Season the roast well with pepper and salt. Let butter foam in a pan, cut the pears in half, remove the core, cut the mandarins in half as well and place them cut side down in the pan, let them cook for 3 to 4 minutes until nicely colored, set aside.
Now sear the roast nicely all around in the same pan and then place it in a baking dish on a branch or leaves of bay. Place in an oven at 170 to 175°C for 35 to 40 minutes. If you use a meat thermometer, the ideal temperature should be between 65 to 70°C for the perfect medium-rare cooking.
Pour off the fat from the pan and deglaze with brown stock, add mustard and cream and season with pepper and salt.
Peel the potatoes, cut into thin slices and then into fine julienne, grating or using a food processor is also fine. Cut the spring onion very finely and mix well with pepper and salt. Take a spoonful of about a tablespoon each time and squeeze out the moisture well. Press into a stainless steel ring and press down firmly, place on a baking sheet, drizzle each rösti with some oil and bake for 25 to 30 minutes.
Add the pears and mandarins in the last 15 minutes with the roast so they cook nicely together. Before serving, slice the roast into 1 cm slices, serve each with a pear, mandarin, the sauce, and rösti.