In a bowl, mix rice flour, cornstarch, and turmeric. Add 1 cup water (see notes) and the coconut milk, and whisk to blend. Stir in green onion.
Set a 12-inch nonstick frying pan (about 10 inches across bottom) over high heat. When pan is hot, add 1 teaspoon oil and tilt to coat bottom.
Stir rice flour batter to blend. Pour 1/2 cup batter into pan all at once and tilt pan to cover entire bottom evenly.
Cook until crêpe is browned and crisp on the bottom, 3 to 5 minutes. With a wide spatula, transfer to a 14- by 17-inch baking sheet. Repeat to cook 2 more crêpes, adding in a single layer to sheet as cooked (use 2 sheets).
Bake in a 350° regular or convection oven until crêpes are crisp, 8 to 12 minutes. Transfer to racks to cool.
Break off pieces and dip into seasoned fish sauce to eat.