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Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with olive oil or butter.
In a medium saucepan over medium heat, warm 1 tablespoon oil or butter. Add sliced onion and cook until softened, about 5 minutes. Stir in garlic and cook 30 seconds more until fragrant.
Sprinkle flour over the onions and garlic. Cook, stirring, for 1 minute to remove raw flour taste. Gradually whisk in milk until smooth. Bring to a gentle simmer, whisking constantly until the sauce thickens—about 2–3 minutes. Remove from heat and stir in cheddar and Gruyère until melted. Season with salt, pepper, and nutmeg.
Arrange half the potato slices in an even layer in the prepared dish. Season lightly with salt and pepper. Pour half the cheese sauce over the potatoes, spreading it evenly with a spatula.
Layer the remaining potato slices on top, season again, and pour the rest of the sauce over. Cover the dish tightly with foil and bake for 30 minutes.
Remove foil, increase oven temperature to 400°F (200°C), and bake uncovered for another 15–20 minutes, until the top is golden and sauce is bubbly.
Let the potatoes rest 5–10 minutes before serving to allow the sauce to set slightly. Garnish with chopped parsley or chives.