This hearty entrée is true comfort food, bringing together many complementary flavors and ingredients. You can make this vegan by substituting vegan sour cream and vegan Parmesan cheese for the dairy versions. You can speed up preparation by using readily available premade mashed potatoes, on sale at most supermarkets.
To make Filling: Cook potatoes in boiling water 15 to 20 minutes, or until tender. Drain, and put into large bowl. Mash, adding sour cream and salt.
Meanwhile, heat 2 Tbs. oil in large skillet over medium-high heat. Add onion, and sauté 8 to 10 minutes, or until golden. Stir in mushrooms, garlic, basil and sun-dried tomatoes, and sauté 2 to 3 minutes more. Stir into mashed potatoes, and set aside.
Chop chard coarsely. Heat remaining 1 Tbs. oil in same skillet over medium-high heat, and sauté until thoroughly wilted, about 5 minutes. Stir chard into potato mixture, and set aside.
Preheat oven to 375F. Spray 9×12-inch baking pan with nonstick cooking spray.
To make Filo: Remove 8 to 10 sheets filo from package, covering remaining filo with plastic wrap. Cut filo to fit pan. Spray every sheet or every other sheet with nonstick cooking spray, or brush with oil as you place them in pan. Spread potato mixture on top of filo layers, and sprinkle with Parmesan cheese. Remove another 8 to 10 sheets filo from package, and top filling with remaining filo, spraying or brushing sheets as before.
Bake about 30 minutes, or until golden. Cut into squares, and serve hot.