Step 1Place the dried mushrooms in a bowl and pour boiling water over. Let stand until softened, about 10 minutes. Lift the mushrooms out of the water into a sieve and rinse well. Squeeze dry. Chop and set aside.
Step 2Melt 4 tablespoons butter in a very large saute pan over medium heat. Add the leeks and a sprinkling of sea salt and cook, stirring, until softened, about 5 minutes. Add the garlic, chopped wild mushrooms and shiitakes and cook, stirring often, until very soft, about 15 minutes. Stir in the tamari and cook until all the liquid is evaporated. Add the thyme and mix well. Remove from heat and cool slightly.
Step 3Heat the oven to 350 degrees. Melt the remaining butter and set aside. Stir the sour cream and cheese into the mushroom mixture, mixing well. Taste and adjust seasoning.
Step 4Combine the bread crumbs, pecans and cayenne and set aside. Brush a 10- by 15-inch baking sheet with butter.
Step 5Lay wax paper on a work surface and brush with melted butter. Spread out 1 sheet of filo dough and brush lightly with butter. (Cover the remaining filo sheets with a damp towel to keep them from drying out.) Repeat with 3 more filo sheets. Sprinkle evenly with the bread crumb mixture. Repeat with the remaining filo sheets.
Step 6Mound the mushroom mixture along the bottom third of the long end of the filo. Fold the short edges in and carefully fold over to enclose the filling completely.
Step 7Carefully transfer the strudel to the prepared pan. Brush with the remaining butter. Bake until browned and crisp, about 40 to 50 minutes. Cool 10 minutes before slicing with a serrated knife to serve.