This southwestern US favourite gets a vegetarian spin with roasted mushrooms instead of pork or chicken. Using canned tomatillos also makes this an accessible recipe no matter where you live in the world. Serve with warmed tortillas (corn or flour will do) for the best experience.
Preheat broiler/grill to high and move the oven rack to the top position. Add the poblanos and jalapeños to a rimmed baking sheet. Place under the broiler and cook until blackened and charred - a few minutes. Remove from the oven and place the peppers underneath an inverted bowl for 10 minutes.
Meanwhile, add the tomatillos and garlic to a rimmed baking sheet and set under the broiler. Cook until charred - about 10 minutes. Set aside. Once cool enough to touch, remove the peels from the garlic.
Move the oven rack to the centre. Switch it to bake mode and reduce the temperature to 220°C/425°F. Add the oyster and shitake mushrooms to a large mixing bowl. Drizzle with 2 tbsp of neutral oil and season generously with salt. Stir to evenly coat then spread over a parchment-lined baking sheet. Roast until golden and reduced in size - about 20-30 minutes - stirring halfway through.
Remove the upside down bowl from the peppers. Peel away the charred skin and discard. Remove the stems and seeds from the poblano. Remove the stem from the jalapeño and the seeds if desired (this will reduce the heat).
Add the poblanos, jalapeños, tomatillos, garlic and ¼ tsp of salt to the bowl of a food processor (see note 1). Pulse several times in order to thoroughly puree the vegetables. Set aside.
Add the remaining 2 tbsp of oil to a large saucepan and set it over medium heat. Once shimmering, add the onion and a generous pinch of salt. Cook, stirring ocassionally, until the onion is softened and transluscent - about 5 mintues.
Add the cumin and chiles de arbol, if using. Stir and cook until very fragrant - about 30 seconds.
Scrape in the pureed tomatillo mixture. Add the roasted mushrooms and pour over the vegetable stock. Stir to combine and bring to a simmer. Allow to simmer for 10-15 minutes, stirring occasionally, just to allow the flavours to mellow and come together.
If using, stir in 2 tbsp of masa harina to thicken the chile (see note 2), adding up to 1 more tbsp to reach the desired consistency. Cook for a minute or two longer, tasting to adjust for seasoning. Remove from the heat and serve.
Notes
If you do not have a food processor, the peppers, tomatillos and garlic can be chopped very finely with a sharp chef's knife. Just note that the finished texture of the chile be a bit more chunky.
Masa harina - which is a Mexican flour made of ground nixtamalised corn - works very well as a thickening agent here. The chile is still good without it, however, if you do have it, it adds a nice thickness and corn flavour. Regular wheat flour cannot be used in its stead as it behaves differently.