Brush or quickly rinse mushrooms and give them time to dry back out. Slice any woody bits from the stems and add those bits to your broth. Slice the mushrooms uniformly, about 1/4-thick.

Set a wide 3-quart pot or Dutch oven over medium heat. When hot, add the oil or butter and then the onions and shallot. Cook, stirring often, until onions are supple and just starting to brown, about 7 minutes. Add garlic and cook 1 minute more.

Add mushrooms, a handful at a time. Let them cook undisturbed for a minute or two, then gently fold them into the onion mixture. Continue until all the mushrooms have cooked down and turned a deep golden brown. Add nutmeg, a big pinch of salt, and black pepper to taste. (Flavors will continue to concentrate as the gravy cooks down, so add salt sparingly.)

Fold in flour and cook, stirring often, 3 minutes. Add a splash of sherry (if using) and soy sauce, scraping up any brown bits. Slowly add broth, a half cup at a time, stirring often and allowing mixture to thicken before adding more. Once all the broth has been added, add bay and thyme, bring mixture to a boil, then turn down to a bubbly simmer and cook 20 minutes, stirring often. Add salt and pepper to taste. Remove bay and thyme and discard.

Before serving, garnish with fresh herbs.

Gravy can be made 3 days ahead and kept in the fridge, or up to 6 months in the freezer.