Preheat the oven to 400°F (200°C) and grease a 8 inch (20 cm) baking dish with butter or coconut oil.
Butter
Place the rhubarb pieces into a bowl with 2 tablespoons of sugar. Mix to coat the rhubarb in the sugar and set aside.
2 cups rhubarb, ½ cup + 3 tablespoons sugar
In a large mixing bowl, whisk the eggs until foamy. Add the milk and vanilla, whisking again to combine, followed by the flour, ½ cup (50 grams) of sugar, and salt. Gently mix until just combined.
4 large eggs, 1 ¾ cups full-fat milk, 1 teaspoon vanilla extract, ½ cup all-purpose flour, 1 pinch sea salt
Pour the clafoutis batter into the prepared baking dish and top with the sugared rhubarb.
Top the clafoutis with the last remaining tablespoon of sugar.
Bake for 40-45 minutes, or until the edges and top of the clafoutis are golden and the middle is just set. It will puff up significantly in the oven and fall when removed, this is normal.
Cool the clafoutis for 15 minutes before serving. Leftovers will keep for a day or two in a sealed container in the refrigerator.