- Preheat oven to 220°C (425°F).
- Cut potatoes in 0.5 cm (1/4 in.) thick slices and boil in large saucepan for about 7 minutes, or until just tender but not fully cooked. Drain and reserve.
- Meanwhile, melt butter in a skillet over medium heat. Cook garlic 1 minute. Sprinkle with flour. Cook, stirring, for 2 minutes.
- Gradually whisk in milk until smooth. Continuing to whisk, bring sauce to a simmer. Reduce heat to low and simmer 3 to 4 minutes, until thickened. Stir in dill, salt, and pepper. Set aside.
- Dust pangasius fillets with cornstarch. Place in a 20 cm (8 in.) square ovenproof dish. Pour one third of reserved sauce onto fish. Arrange potato slices on top and spoon on remaining sauce. Bake until surface is golden and fish is cooked through, about 20 minutes.
Replace pangasius by haddock, sole, or pollock.