Preheat the oven to 180C/360F. Rinse the mushrooms thoroughly under running tab water without submerging them, and pat dry immediately. Carefully hollow the inside of the caps of the mushrooms, saving the stems. Slightly salt the caps and put a small dice of butter in every cap (saving 1tbsp of it). Place them in a baking dish and bake for 10 minutes. Meantime chop finely the spring onions, garlic and mushroom stems. Peel and shred the potato. In a frying pan over medium heat melt the rest of the butter and add the vegetables. Season with some salt and pepper and let them sweat for 10-15 minutes, stirring occasionally, until the potatoes are almost cooked and all the vegetables are soft. Stirring, add the cream and leave for just one minute. Remove the vegetable filling from heat and allow cooling for 5 minutes. Take of the mushroom caps and cool them slightly too. When the vegetable mixture has cooled enough check the seasoning and carefully spoon into every mushroom cap. Return the stuffed mushrooms in the oven for 10-15 more minutes, until the filling is golden. Let the mushrooms cool slightly before serving.