Preheat the oven to 220°C fan/240°C. Put all the ingredients for the roasted mushrooms into a 30 x 20cm non-stick, high-sided baking tray and mix well. The size and height of the tray is important here – if it’s any bigger and not high-sided, you won’t create the environment you need to make the mushroom gravy. Roast for 15 minutes, then remove from the oven.
Transfer the mushrooms to a sieve set over a medium saucepan, so all the liquid – which will be your gravy – drains into the saucepan. Help things along by giving the mushrooms a really good squeeze. You should end up with about 110g of liquid in the pan, but if you have any less, supplement with equal amounts of soy sauce and water. Set the saucepan aside until later.
Return the squeezed mushrooms to the tray, spread out, and roast for another 5 minutes. Set aside to cool completely. Switch the oven off, leaving the door open so it cools down.
Put all the ingredients for the dough into the bowl of a stand mixer and knead on medium-high speed for 10 minutes. If you don’t have a mixer, bring the ingredients together in a bowl, then transfer to a work surface lightly greased with olive oil and knead for 15 minutes. The resulting dough should be smooth and pliable.
Tip the dough on to a lightly oiled surface. Lightly grease your mixing bowl with olive oil. Shape the dough into a smooth, round mass and return it to the bowl. Lightly grease the surface of the dough with some olive oil to stop it drying out, then cover with a damp cloth. Place the bowl in the cool, switched-off oven (make sure it’s only just warm and not still hot, otherwise wait a bit longer before placing the bowl in the oven). Close the door and leave to prove for 1 hour, or until doubled in size.
Very finely chop the roasted mushrooms to a mince consistency – you can easily do this by pulsing a few times in a food processor, if you have one. Set aside.
Place a large piece of non-stick parchment paper on your work surface and lightly grease it with olive oil. Tip the proved dough on to your oily paper. Use your hands to press the dough out into a 35 x 23cm rectangle; the dough will spring back at first, but keep on stretching it into a rectangle until it stops springing back. Make sure the thickness of the dough is as even as possible, and patch up any holes that form by pinching the dough back together. Scatter the mushrooms evenly over to cover the dough.
Starting at one of the shorter ends, use the oily paper to help you roll the dough up like a Swiss roll, making sure you end up with the seam of the dough underneath. Tuck the dough in and under at either end of the roll, so the filling doesn’t spill out. Use the parchment to help you lift the roll on to a flat baking tray, again ensuring that the roll is seam side down. If there are any holes in the dough, pinch them together.
Return the roll uncovered to the cool, switched-off oven, close the door and prove for another 30 minutes, or until significantly puffed up.
Remove the tray from the oven and set aside. Preheat the oven to 180°C fan/200°C.
For the wash, mix the milk and oil together. Brush the roll all over with the milk mixture, then sprinkle the sesame seeds to cover the roll, pressing them into the surface so they stick.
Once the oven is hot, bake the roll for 30 minutes, rotating the tray halfway so it colours evenly. Lower the heat to 170°C fan/190°C and bake for another 10 minutes until golden brown. The surface of the roll will feel quite hard to the touch at this point, but it will soften as it cools. Remove from the oven and leave to cool for 10 minutes. If you have one, use a blowtorch to lightly char the sesame seeds. Sprinkle with flaked salt.
Mix all the ingredients for the salsa together.
Place the pan with the mushroom gravy on a medium heat and cook until heated through. Give it a good mix to combine the oil and the mushroom juices.
Cut the roll into thick slices and serve with the salsa verde and mushroom gravy.
Make Ahead
Roast the mushrooms, pour off the gravy and then chop the mushrooms up to 2 days before. Store the chopped mushrooms and the mushroom gravy in the fridge separately, bringing them back to room temperature while the dough proves. If you’ve got ahead by roasting the mushrooms, you’ll need to preheat the oven to the lowest temperature (no higher than 50°C) before switching off the oven and proving the dough inside it.
Make the dough a day ahead and, after the first prove, keep refrigerated in a large, sealed container. Bring back to room temperature before stretching out into a rectangle and filling it.
