In a heatproof bowl, combine the dried porcini mushrooms and boiling water. Soak for 20 minutes or until soft.
Preheat the oven to 180°C, 350°F, Gas 3. Place the chicken or vegetable stock, whichever you are using, in a saucepan over a medium-low heat and bring to just under a simmer.
Add the butter to a large saucepan or casserole dish with a lid, and place on a medium heat. Add the chopped onion and the chopped garlic and cook for 6-8 minutes until soft.
As the onion is cooking, drain the porcinis but reserve the soaking liquid. Roughly chop the porcinis and set aside, then strain the soaking liquid and add to the simmering stock. Season this liquid with salt and pepper.
Add the chopped porcinis to the onion and cook, stirring frequently, for 1-2 minutes. Next, stir in the risotto rice and cook for two minutes, stirring gently. Add in the white wine, stir and cook for 2-3 minutes until it has evaporated. Pour in the simmering stock, stir to combine, place a lid on top and put it into the oven. Cook for 10-12 minutes until just al dente.
Remove the risotto from the oven, add the grated Parmesan and cubes of butter, then use a wooden spoon to beat everything together. Stir in the marjoram or parsley, whichever you are using, lemon juice and mascarpone, if using.
Serve immediately with grated Parmesan on top.