Preheat the oven to 180°C (356°F). Line the baking tray with baking paper.

Heat the olive oil in the frying pan. Brown the spring onion cut-side down.

Add 15 g butter and let it foam around the onion.

Pour in 100 ml water and add the miso paste with a pinch of salt.

Simmer until the liquid reduces. Turn the onion until glossy and very tender.

Melt the remaining 15 g butter in the saucepan. Stir in the wheat flour.

Cook the roux for 1 minute. Gradually whisk in the 300 ml milk.

Cook until the béchamel thickens. Mix in the béchamel spice blend, salt, and Dijon mustard.

Roll out the puff pastry if needed. Cut four large pastry rounds.

Mark a smaller circle in the center of each round without cutting through. Transfer the rounds to the baking tray.

Brush only the pastry borders with egg yolk. Sprinkle the borders with sesame seeds.

Spoon the béchamel into each pastry center. Place one glazed spring onion half on each tartlet.

Bake until the pastry is puffed and deeply golden, about 20 minutes. Serve warm.