Preheat and grease the dish.
Heat your oven to 375°F (190°C). Lightly grease a 9-inch (23 cm) baking dish with a bit of olive oil or cooking spray.

Par-boil the potatoes.
Bring a pot of salted water to a boil. Add the thinly sliced potatoes and cook for 6 minutes—they should be slightly tender but not falling apart. Drain well in a colander and let the steam escape for a minute so they don’t water down the custard.

Cook the leeks.
While the potatoes drain, heat 1 tbsp (15 ml) olive oil in a skillet over medium heat. Add the sliced, rinsed leeks and cook for about 5 minutes, stirring often, until softened and fragrant (they should look silky, not browned).

Layer the vegetables.
Arrange the potatoes and leeks in the prepared baking dish, layering and lightly spreading them out so every slice gets some leek.

Mix the egg custard.
In a bowl, whisk together 5 eggs, 1 cup (240 ml) milk, 1 tsp dried thyme, 3/4 tsp salt, and 1/4 tsp black pepper until fully combined and a little frothy.

Assemble.
Pour the egg mixture evenly over the potatoes and leeks. Sprinkle the top with 1 cup (100 g) shredded Swiss or cheddar.

Bake until set and golden.
Bake for 30–35 minutes, until the top is golden and the center is set. A good cue: the middle should no longer look wet or jiggly, and a knife inserted near the center should come out mostly clean (a little moisture is fine, but it shouldn’t be liquid).

Rest before slicing.
Let it rest for 10 minutes. This helps it finish setting and makes cleaner slices.