Place the cabbage parts in the oven dish, and cover with 5 tbsp of olive oil, salt & pepper. Roast for at least 45 minutes.
Cook the potatoes until soft. The cooking time depends on the potato size.
In the meanwhile, you can make the mushroom gravy already. Roast the shiitakes in some olive oil on a medium high heat.
Mix the cornstarch with the cold stock and the maggi (or another flavor enhancer). When the mushrooms are browned, add the stock mixture. Bring this mixture to a boil until it begins to thicken. To make sure all the lumps are out, I love to use a whisk for this.
*In love with butter? After roasting the shiitakes, you can add the stock and maggi. Cook this until it has reduced about a quart. Then take the pan of the heat, and add the cubes of butter while whisking with a whisk. Add the next cube of butter, when the first one is completely dissolved in the sauce.
Chop the chives in small pieces and add these to the sauce.