Prepare the Umami Stock: Place the dried porcini mushrooms and 1 teaspoon of Marmite, soy sauce, and miso paste into a measuring cup. Pour over 2½ cups of boiling water and leave it to steep into a rich dark stock.
Boil the Potatoes: Place the chopped potatoes into a large pot of salted cold water. Bring to a boil and cook for 15 minutes or until completely fork-tender, then drain.
Build the Flavor Base: Heat 2 tablespoons of olive oil in a large skillet or Dutch oven over medium heat. Sauté the chopped rosemary and garlic for 1–2 minutes until fragrant. Add the fresh chopped mushrooms, ground coriander, and a pinch of black pepper. Cook for 5 minutes until the mushrooms begin to soften.
Caramelize the Frozen Veg: Dump the entire bag of frozen pre-chopped celery, carrots, and onions straight into the skillet. Cook this mixture down over medium-high heat for 10 to 15 minutes. Let the moisture evaporate so the vegetables genuinely brown, deeply caramelize, and concentrate in flavor.
Deglaze and Simmer: If using wine or beer, pour it in now and let it bubble away to reduce by half. Pour in the can of lentils and the can of chickpeas (including all the starchy aquafaba liquid from the cans to help thicken the gravy). Add the chopped sun-dried tomatoes and the prepared mushroom-Marmite stock (including the rehydrated porcini mushrooms).
Mash the Ragu: Bring the mixture to a simmer. Take a potato masher and gently crush the mixture directly in the pan a few times. This breaks up the whole chickpeas and creates a thick, chunky texture that perfectly mimics a traditional meat-style filling. Let it simmer gently for 10 minutes until thick and glossy.
Make the Potato Topping: Mash the drained potatoes thoroughly with 2 tablespoons of extra virgin olive oil, a pinch of salt, pepper, and a grating of fresh nutmeg until smooth and rustic.
Assemble: Pour the thick vegetable and chickpea ragu into a large 9x13-inch baking dish. Spoon the olive oil mash evenly over the top, using a fork to scuff up the surface to create plenty of rough edges.
Crispy Finish & Bake: Strip the fresh thyme leaves into a small bowl with 1 tablespoon of olive oil, mix it around, and drizzle this herb oil over the top of the mash. Bake in a preheated oven at 400°F for 35 to 40 minutes until the gravy is bubbling up around the sides and the potato edges are golden and crispy.