This dish is very tasty served cold with a drizzle of cold Lemon Sauce, so it can be made ahead for a fancy luncheon or served as leftovers at brunch the next day.
In a large skillet over medium-low heat, place salmon fillet and sherry. Cover and poach 15 minutes, or until fish is cooked through and flakes. Remove salmon; cool, cover, and refrigerate. Discard sherry.
Prepare Parmesan Crust. While crust is baking, melt butter in a medium skillet over medium heat. Add chopped onion and garlic; saute until the onion is soft, about 5 minutes. Remove from heat. In a large bowl, beat the sour cream and eggs with a wire whisk until well mixed. Flake the poached salmon fillet into bite-sized pieces and fold into the sour cream mixture. Add the onion mixture, 1 cup Gruyere cheese, dillweed, salt, and pepper. Stir well to combine.
Pour salmon mixture into prepared crust and top with remaining ½ cup cheese. Bake uncovered at 375° 1 hour, or until cheese topping is browned and firm. Remove from oven and allow pie to rest 5 minutes. Prepare Lemon Sauce.
Invert salmon loaf pie onto a serving platter. Garnish with parsley sprigs. To serve, cut pie into thick slices and drizzle each slice with Lemon Sauce.
Preheat oven to 375°. In a medium bowl, combine flour and Parmesan cheese. Cut in butter until it is the size of small pebbles. Slowly sprinkle crust mixture with 3 tablespoons water; mixture should resemble coarse meal. Press crust into an ungreased 2-quart nonstick loaf pan, covering the bottom and sides, with a lip of crust folded over the edge of the pan. Bake 15 minutes, or until golden brown. Cool.
Combine cornstarch and 1-½ teaspoons water in a small bowl; stir to dissolve cornstarch. In a small saucepan over high heat, bring 3⁄4 cup water to a boil. Slowly whisk in cornstarch mixture. Boil 1 minute, then remove saucepan from heat. In a medium bowl, beat egg and egg yolk with a wire whisk until light and fluffy. Whisk in lemon juice and salt. Pour hot cornstarch liquid into egg-and-lemon mixture very gradually, whisking constantly until well mixed.