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 oil and butter 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, 5 minutes.