In a small saucepan over medium heat, warm the olive oil and sauté the minced garlic for about 60 seconds until fragrant. Add the oat milk, vegetable broth, nutritional yeast, smoked paprika, thyme, salt, pepper, and the pinch of nutmeg if using. Whisk everything together and bring to a gentle simmer for about 2 minutes — don't let it boil. Taste and adjust the salt.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13 baking dish with olive oil. Pat your potato slices dry with a clean kitchen towel — this prevents a watery bake.

Start layering the thinly sliced potatoes in the prepared dish, overlapping them slightly like roof shingles. After every two layers of potato, add a layer of sliced onions. Season lightly with salt and pepper between layers.

Pour the warm sauce evenly over the layered potatoes. Press down gently with the back of a spoon so the liquid starts to soak in. Cover the dish tightly with aluminum foil.

Bake covered at 375°F (190°C) for 45 minutes.

Remove the foil and bake for another 25 to 30 minutes, until the top is deeply golden, the edges are slightly crispy, and a knife slides through the center without any resistance. For extra browning, broil for 3 to 4 minutes — watch it closely.

Let the bake rest for at least 10 minutes before slicing and serving. This allows the sauce to thicken and the layers to set. Garnish with fresh parsley or chives and serve hot.