Preheat your oven to 350°F (175°C). Lightly grease a 9-inch (23 cm) round baking dish with butter. A shallow ceramic or glass dish works well because it allows the custard to cook evenly.
Wash the fresh blackcurrants and remove any stems. Pat them dry with a clean kitchen towel. If using frozen berries, thaw them completely and drain away excess liquid.Spread the blackcurrants evenly across the prepared baking dish. Sprinkle with the optional tablespoon of sugar and lemon zest if you prefer a slightly sweeter, brighter fruit layer. Place the white miso paste in a mixing bowl with a small amount of the milk. Whisk until the miso dissolves and the mixture becomes smooth. This step helps prevent salty miso lumps from forming in the finished custard.
In a separate bowl, whisk the eggs and sugar until the mixture is smooth and slightly pale. There is no need to whip it to the volume of a sponge cake; the goal is to create a uniform custard base.
Pour in the remaining milk, heavy cream, dissolved miso, vanilla extract, and melted butter. Whisk gently until everything is thoroughly combined.
Sift the flour into the wet mixture and whisk until no visible lumps remain. The batter should be relatively thin, similar to pancake batter. Avoid overmixing, which can make the texture less delicate.
Pour the custard batter evenly over the blackcurrants in the baking dish. Gently shake the dish to distribute the batter around the fruit. A few berries may remain visible on the surface, creating an attractive baked finish.
Place the dish in the preheated oven and bake for 35–45 minutes, or until the edges are golden and the center is just set. A slight wobble in the very center is fine, but the batter should not look liquid.If the top browns too quickly, loosely cover it with foil for the remainder of the baking time. Remove the clafoutis from the oven and allow it to rest for 10–15 minutes. It will puff up while baking and settle as it cools. This is normal and gives the custard time to firm up for easier serving.
Dust lightly with powdered sugar just before serving. Add a few fresh blackcurrants or mint leaves for decoration if desired. Serve warm for a soft, pudding-like consistency, or refrigerate after cooling for a firmer custard texture.