In a small bowl, combine the dried morel and porcini mushrooms.
Pour the boiling water over them and set aside to rehydrate for at least 20-30 minutes.
In a Dutch oven or large heavy-bottomed pot, heat 1 Tbsp (15 ml) of olive oil over medium heat.
Add half of the fresh cremini, portobello, and shiitake mushrooms.
Cook, stirring only occasionally, until deeply browned, about 5-7 minutes.
Transfer the cooked mushrooms to a separate bowl.
Repeat with another 1 Tbsp (15 ml) of olive oil and the remaining fresh mushrooms.
Reduce the heat slightly and add the remaining ½ Tbsp of olive oil to the pot.
Add the leek and carrots, and cook until they begin to soften and lightly brown, about 5 minutes.
Stir in the minced garlic and cook for another minute until fragrant.
If using, pour in the Cognac and use a wooden spoon to scrape up any browned bits from the bottom of the pot.
Let it cook for a minute until the alcohol smell dissipates.
Carefully strain the liquid from the rehydrated mushrooms into the pot, leaving any grit behind. Finely chop the rehydrated mushrooms and add them to the pot.
Pour in the mushroom broth, then stir in the Dijon mustard, vegan Worcestershire, liquid aminos, tomato paste, bay leaves, parsnip, rutabaga, celery root, fresh herbs, and black pepper.
Increase the heat to bring the stew to a boil.
Once boiling, reduce the heat to low, cover the pot, and let it simmer for 25 minutes, or until the root vegetables are tender.
Stir in the frozen pearl onions, replace the cover, and cook for an additional 5 minutes.
Remove the pot from the heat, discard the bay leaves, and stir in the aged sherry vinegar.
Ladle the stew into warm bowls.
Garnish with a few drops of truffle oil, if desired, and serve immediately.
