Preheat oven to 375°. Spray a 2-quart oval baking dish with cooking spray. Place on a parchment paper-lined baking sheet.
In a large saucepan, bring all potatoes, 1½ tablespoons salt, and water to cover to a boil over medium-high heat. Reduce heat to medium-low; cook until potatoes are almost tender, about 5 minutes. Drain; rinse with cold water to stop the cooking process.
In a Dutch oven, heat 2 tablespoons butter over medium heat. Add fennel, onion, and garlic; sauté for 4 minutes. Add mushrooms; cook until vegetables are tender, 3 to 5 minutes, reducing heat as necessary to prevent overbrowning. Stir in honey and vinegar; cook until mostly reduced, 4 to 5 minutes. Add remaining 4 tablespoons butter. Sprinkle vegetables with flour; cook for 1 minute, stirring constantly. Gradually stir in milk; bring to a boil. Reduce heat, and simmer, stirring frequently, until thickened, 5 to 8 minutes. Add gorgonzola, sour cream, thyme, rosemary, pepper, and remaining 1½ tablespoons salt. Stir in potatoes; transfer to prepared dish.
On a lightly floured surface, roll one piecrust into an oval to fit baking dish; cut lengthwise into 1-inch-thick strips. Repeat with remaining piecrust, cutting dough crosswise into 1-inch-thick strips. Arrange strips in a lattice design over filling. Trim strips to inside edge of baking dish, and press ends onto sides. Brush dough with egg white.
Bake until crust is golden brown and filling is bubbly, 40 to 45 minutes. Let cool for 20 minutes. Garnish with sea salt, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/mixed-potato-pot-pie/