Heat 3 tablespoons butter and 2 tablespoons oil in a large saute pan over high heat. Add the mushrooms, season with salt and pepper, to taste, and saute until golden brown.
In a medium saucepan heat remaining butter and oil over medium heat, add the onions and cook until soft. Add the rice and stir to coat with the butter and oil. Add 6 cups of the stock, 1 teaspoon of salt and the porcini powder, bring to a boil, cover, lower the heat and cook until all the liquid is absorbed. Fold in the mushroom mixture, more stock, if needed, and season with salt and pepper, to taste. Cook to heat through, about 5 minutes.