Preparation

Place a rack in the bottom third of the oven and preheat to 375°F. Melt the butter in a large skillet over medium heat. Add the leeks, sage, and 1/2 tsp. salt. Cook, stirring often, until softened and golden brown, about 10 minutes. Transfer to a large bowl.

Return the skillet to medium-high heat and add 1 Tbs. olive oil. Add half of the mushrooms and 1/2 tsp. salt. Cook, stirring occasionally, until softened and golden brown, 3 to 4 minutes. Transfer to the bowl with the leeks. Repeat with the remaining oil, mushrooms, and 1/2 tsp. salt.

Gently fold the walnuts, heavy cream, parsley, and pepper into the mushroom mixture. Season to taste with salt.

Working with one sheet of phyllo at a time (keep remaining sheets covered with a clean kitchen towel), arrange the dough in a lightly greased 8- to 9-inch square baking pan. Brush the sheet lightly all over with olive oil. Repeat with remaining phyllo sheets, rotating the pan slightly after each addition and fanning out the layers, allowing the excess to hang over the edges to form a crust. Spoon the mushroom mixture evenly into the pan, then gently press the dough in to create a crust while letting it spill around the edge.

Bake in the bottom third of the oven until the crust is deep golden brown around the edges, 25 to 30 minutes. Sprinkle with sumac, if using.