2-1/2 lb (1.1 kg) Yukon Gold potatoes, (about 7), peeled and halved crosswise
2 tbsp (25 mL) butter
1-1/2 cups (375 mL) sliced leeks, (white and light green parts only)
3 tbsp (45 mL) all-purpose flour
1/2 tsp (2 mL) salt
1/4 tsp (1 mL) pepper
1 pinch dried thyme
1-3/4 cups (425 mL) milk

Topping:
1/2 cup (125 mL) fine fresh bread crumbs
2 tbsp (25 mL) grated parmesan cheese
1 tbsp (15 mL) butter, melted

In large pot of boiling salted water, cover and cook potatoes until almost tender, about 10 minutes; drain and let cool. Cut crosswise into 1/4-inch (5 mm) thick slices.

Meanwhile, in large saucepan, melt butter over medium heat; cover and cook leeks, stirring occasionally, until tender, about 7 minutes.

Stir in flour, salt, pepper and thyme; cook, stirring, for 1 minute. Gradually whisk in milk; cook, whisking, until thick enough to coat back of spoon, about 5 minutes. Remove from heat. Gently stir in potatoes to coat. Spread in 8-cup (2 L) casserole dish or 8-inch (2 L) square glass baking dish. (Make-ahead: Cover and refrigerate for up to 24 hours.)

Topping:
In small bowl, stir together bread crumbs, cheese and butter until crumbly; sprinkle over potatoes. Bake, uncovered, in 425°F (220°C) oven until heated through and topping is browned, 20 to 25 minutes.

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