Preheat the oven at 180°C fan forced / 200°C conventional.
Soak the porcini mushrooms in warm water. Lift them out of the water to leave any grit behind, gently squeeze them, discard the first soak water and replace it, and soak again until fully rehydrated and thoroughly squeeze out the soaking water.
Finely chop the porcini mushrooms.
Trim and halve or quarter the Swiss brown mushrooms. Place them on a baking paper-lined tray, toss with a little extra virgin olive oil, and spread them out cut-side down.
Season with truffle salt and white pepper then roast them tossed with olive oil, salt, and white pepper.
Roast for 20–25 minutes or until the liquid has evaporated and the mushrooms are golden on the underside.
Cool, then slice to about 5–7mm thick.
Remove and discard the stems from the shiitake mushrooms. Halve any large ones, then cut the rest into 5–7mm slices.