Preheat the oven to 400 degrees. In a bowl, add 1 cup of boiling water to the sun dried tomatoes and put to the side. In an ovenproof 10 inch skillet, heat the oil over medium low heat for 1-2 minutes, until oil is warm but not hot. Add chopped onion and garlic and saute for 2 minutes. Add red pepper flakes and saute for an additional minute. Add the yam and shiitake mushrooms and saute for 10 minutes. Add the remaining vegetables in the order listed above with about 1 minute between additions. Saute all the ingredients together for 4 minutes more or until vegetables have had a chance to soften, but no lose their firmness. While vegetables are sauteing, lightly beat eggs, milk, salt and pepper in a mixing bowl. Once vegetables have reached desired firmness, shake the skillet to evenly distribute vegetables. Add egg mixture, allowing it to flow to all the edges of the skillet. Do not stir. After about a minute, sprinkle feta all around the cooking frittata. Cook for one last minute or until you see the eggs begin to set around the edges. Slice up the soaking tomatoes and place them on top of the frittata just before transferring it to the oven. Then put the pan in the heated oven and bake until the eggs are puffed and set, approximately 12 minutes longer. Remove from the oven, cut into wedges and serve hot.