For the mushrooms, in the states, White Button would be preferred, prep them by cutting the stems off even with the caps, gives a nicer appearance, but I would use the stems with this as well as they are just as tasty. Just toss them in when no one is looking. For Asia, Straw Mushrooms would be preferred but I would cut each mushroom in half lengthwise. Now that we have the mushrooms covered, let's continue.
Preheat your oven to 190 C (375 F). Place the prepped mushrooms in a small baking dish in a single layer.
In a bowl, mix together the butter, mustard, onion, salt, cayenne pepper, nutmeg, and flour. Spread this over the mushrooms, not looking to be fancy here wit this or win any photo contests.
Pour the cream over the mushrooms and then sprinkle the mushrooms with the white pepper.
Place the baking dish on a rimmed baking sheet and bake for 45 minutes, stirring 1-2 times during the baking.
Remove from oven, let rest (cool) for a few minutes, serve hot as a side dish or appetizer. Enjoy.