Preheat oven to 425°F (220°C). Prick potatoes all over with fork. Microwave on HIGH until fork-tender, about 15 to 18 min.
Meanwhile, melt 3 tbsp (45 mL) butter in large Dutch oven set over medium heat. Cook leeks, carrots, celery and garlic, stirring occasionally, 5 to 7 min. until softened. Stir in flour; cooking 1 min. Whisk in broth, 1/2 cup (125 mL) water and 3/4 cup (175 mL) milk until sauce is smooth. Cook until thickened, 2 to 4 min. Remove from heat; stir in Supreme Seafood Mix, dill, mustard and 3/4 tsp (4 mL) each salt and pepper. Set aside.
When potatoes are cool enough to handle, peel and mash with remaining butter, milk, salt and pepper until smooth.
Spoon seafood mixture into greased 13 x 9-in (3-L) baking dish (mixture will be slightly runny). Spoon potato mixture over top; spreading to edges of dish. Use a fork to lightly drag tines over potato mixture to create a pattern. Set baking dish on rimmed baking sheet and bake until filling is bubbling at edges and top is lightly golden, 25 to 30 min. Broil 2 to 3 min. until top is golden brown. Garnish with chives before serving.
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