If the mushrooms are small leave them whole. Otherwise, cut them through the stems into halves or quarters so they are a uniform size.
Put a pan of water on to boil. When it is boiling add the mushrooms to the water and blanch for 2 minutes. Blanch just means boiling in water and it helps to preserve their colour, flavour and texture.
Drain the mushrooms from the water using a colander and arrange the mushrooms in an oven-proof dish.
Preheat the oven to 170, Gas 3, 325 F
Peel the onion and dice it into small pieces.
Next, heat the frying pan over a medium heat. When the pan is hot, add the olive oil (tbsp). When the oil is hot add the diced onion
Fry, stirring often, until softened and translucent.
Add the onions to the casserole dish with the mushrooms.
Now add the peppercorns (5), bay leaves (2), chicken stock (150ml), wine and tomato puree (2tsp) into the oven-proof dish. Season with salt and pepper.
Stir the ingredients to mix the sauce.
Cover the dish and cook in the oven for 35 - 40 minutes. The sauce should thicken.
Remove from oven and leave to cool. Remove the bay leaves and peppercorns before serving.