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.
Substitute cod with haddock or pollock.