Preheat your oven to 350°F (175°C). Generously butter a 9-inch deep-dish pie plate or similar-sized baking dish.
Wash the rhubarb stalks thoroughly and trim away any leaves. Cut the stalks into approximately ½-inch pieces. Arrange the chopped rhubarb evenly across the bottom of your prepared baking dish.
In a medium bowl, whisk together the eggs until light and frothy (about 1 minute). Gradually add in the sugar while continuing to whisk until the mixture becomes pale and slightly thickened.
Sift together the flour, nutmeg, and salt before gently folding them into your egg mixture.
Slowly whisk in the milk, vanilla extract, and melted butter until fully incorporated. The mixture should be smooth and pour easily.
Pour the custard mixture over the rhubarb, allowing it to settle evenly throughout the dish. If using, sprinkle turbinado sugar across the top.
Bake in the preheated oven for 50-55 minutes, or until the center is just set with a slight wobble (internal temperature should reach 175°F).
Allow the pie to cool on a wire rack for at least 30 minutes before serving. The custard will continue to set as it cools.