Start by making the dough. Combine the dry ingredients in a bowl, then add the water little by little and bring the dough together using your hands; you should have a sticky ball. Turn it out onto a floured surface and knead for 5 minutes, until smooth and bouncy, then place in an oiled bowl. Cover with a kitchen towel and leave in a warm place to double in size for 1 to 1½ hours.

Meanwhile, put the vinegar for the pickled cucumber into a small saucepan with 3 tablespoons of water. Bring to a simmer, then pour into a bowl, add the cucumber, and leave to cool.

Turn the dough out onto a floured surface, knead for a minute to knock out the air, then divide into 10 equal pieces. Take one piece, flatten it into a ½-inch-thick disk, then brush one half with a little oil. Fold the bun into a half-moon and place on a small square of parchment paper on a tray. Repeat with the remaining dough, then loosely cover the tray with a kitchen towel and leave to rise for 30 minutes more.

Now for the filling. In a small bowl, whisk the soy sauce, peanut butter, vinegar, garlic, and sesame oil. Heat the canola oil in a frying pan on a high flame, then fry the mushrooms for 6 minutes, until soft and browning at the edges. Stir in the sauce to coat, then turn the heat to medium and cook, stirring regularly, for 5 minutes, until the sauce reduces and darkens.

To cook the bao, set a steamer over a pan of simmering water. Put the bao, still on their parchment-paper mats, into the steamer in batches, making sure they don’t touch. Cover and steam for 8 minutes.

Once done, fill each bao with a generous tablespoon of mushrooms, 3 or 4 slices of cucumber and, for a little crunch, some peanuts.