Combine the flour, salt, and sugar in a mixing bowl. Add the cubed butter and use a pastry cutter to work the butter into the dry ingredients. When butter has reached pea size, add the ice water all at once and use your hands to knead the dough. Gather into a ball and shape into a disc. Wrap in plastic and chill in refrigerator while you make the filling.

Preheat oven to 400°F. Lightly grease a 10-inch deep-dish pie plate and set aside.

Bring the vegetable stock to a boil in a medium pot over high heat. Turn off heat and add the salmon cubes. Let sit for 5 minutes, stirring occasionally to ensure even cooking. Use a slotted spoon to transfer salmon to a bowl. Do not discard stock.

Melt the butter in a large-rimmed pan over medium heat. Add the shallots, fennel, salt, and pepper. Sauté for 5 minutes until soft. Stir in the flour and cook for 1 minute. Whisk in the stock used for poaching the salmon along with the heavy cream. Bring to a boil and remove from heat. Stir in the corn, peas, and onions. Gently fold in the salmon chunks. Transfer the filling to the prepared pie plate.

Roll out the dough on a lightly floured surface to a 12-inch circle and lay over the filling. Trim and flute the edges and cut three slits on top. Brush top of dough with egg white. Bake pot pie for 30 minutes. If desired, you can finish the pie under the broiler to brown the crust. Remove from oven and let sit for at least 5 minutes before serving.