DO-AHEAD TIP: Bake potatoes.
If you haven’t already done so, bake the potatoes.
Preheat oven broiler. Place dried Shitake mushrooms and water in a saucepan with a tight-fitting lid; bring to a boil then reduce heat, cover and simmer for 10 minutes or until mushrooms are rehydrated; remove from heat and set aside.
Meanwhile, heat the oil in a skillet over medium-high heat; add onion and garlic and sauté until onion is translucent. Add sliced Portabella mushrooms and cook for about 5 minutes.
Drain Shitake mushrooms and slice; add to the skillet along with the button mushrooms; salt and pepper to taste and cook for 3 minutes.
Re-heat baked potatoes then cut in half lengthwise. Spoon mushroom mixture over potato halves; top with shredded cheese, marjoram and chopped green onions. Broil until cheese melts.