Preheat oven to 180°C/350°F. Add the porcini mushrooms to a small bowl and pour over the boiling water. Cover the bowl and allow the mushrooms to soak for about 20 minutes. Drain the mushrooms, reserving the liquid, and roughly chop them. Set aside.
Meanwhile, add the oyster mushrooms to a parchment-lined baking tray (no oil, salt or pepper required) and move to the oven. Roast until reduced significantly in size and browned in places, about 20 minutes.
While the mushrooms are roasting, add the olive oil to a large saute pan or skillet and set over medium-high heat. Once shimmering, add the onion along with a generous pinch of salt. Cook, stirring every so often, until they're lightly caramelised - about 15 minutes.
Add the garlic, miso paste and tomato paste to the onions and stir to combine. Cook for 2-3 minutes before adding in the flour and cooking for one more minute.
Stir in the roasted oyster mushrooms and the chopped porcini mushrooms. Pour over the vegetable stock, reserved mushroom soaking liquid, mustard and sherry vinegar. Stir to combine and bring to a simmer.
Allow the stroganoff to simmer for at least 5 minutes, just to allow the flavours to meld and the sauce to thicken slightly. Pour over the coconut milk along with a few generous grinds of black pepper. Simmer for another 5 minutes. Taste to adjust for seasoning, adding more salt and pepper where needed.
Serve, spooning the stroganoff over cooked egg noodles, rice or buckwheat groats - or simply enjoy plain.
Notes
Any additional coconut milk can be stored in an airtight container in the fridge for up to 5 days or frozen for up to 3 months.