Place the chicken roll on a sheet of baking parchment with the wine, thyme and the garlic clove. Wrap the parchment into a parcel and then enclose it in aluminium foil so the moisture stays inside.
Bake in the oven (fan setting) at 190 °C for 45 minutes. Open the foil and bake for 15 minutes more, turning once, until the roll is nicely browned.
Beat together the mustard, soy sauce and the juice of the lemon until smooth. Boil the potatoes (cut in cubes), then mash with a whisk or a potato masher. Add a little butter and milk, season with salt and pepper.
Transfer the mash to a bowl, rinse the pot, put it back over the heat with a little olive oil and sauté the mushrooms and pepper. Take the pot off the heat and put the mash back in.
Serve the chicken roll on a wooden cutting board and accompany with the garnish served in small bowls.
