Place potatoes in large saucepan; cover with cold water. Bring to a boil. Reduce to a simmer and cook 20 to 25 min., or until just tender. Drain. Cool until comfortable enough to handle. Slice into 1/4-in. (5-mm) thick rounds.
Meanwhile, preheat oven to 425°F (220°C). Melt butter in high-sided skillet over medium heat. Cook garlic 1 min., until softened. Sprinkle with flour. Cook, stirring 2 min., then gradually whisk in milk until smooth. Continuing to whisk, bring just to a boil. Reduce to a simmer 3 to 4 min., until sauce is thickened. Stir in dill and salt. Set aside.
Dust cod with cornstarch; place in 7x11-in. (2 L) baking dish. Arrange potato slices on top of fish. Pour sauce over top. Bake 30 to 35 min. until surface is golden and fish is cooked through.
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