Roll out pastry on floured work surface to 12-inch square. Transfer to 9-inch-diameter glass pie dish. Trim overhang to 1 inch. Fold under; crimp edges. Cover; chill. Melt butter in large nonstick skillet over medium-low heat. Add leeks and thyme. Sprinkle with salt and pepper. Cover; cook until leeks are very tender but not brown, stirring often, about 10 minutes. Add chard; saut until wilted, about 2 minutes. Remove from heat; cool. Position rack in bottom third of oven; preheat to 425F. Whisk cream and next 5 ingredients in large bowl. Mix in cooled leek mixture. Pour filling into crust. Bake tart 15 minutes. Reduce heat to 350F and bake until filling is puffed and just set in center, about 15 minutes longer. Transfer to rack; cool 10 minutes. Makes 8 servings. Bon Appetit October 1999 Per serving: 1715 Calories (kcal); 157g Total Fat; (81% calories from fat); 33g Protein; 50g Carbohydrate; 1456mg Cholesterol; 2732mg Sodium Food Exchanges: 0 Grain(Starch); 3 Lean Meat; 7 1/2 Vegetable; 0 Fruit; 29 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.