To make the stock: Fill a four -quart pot more than halfway with water. Add the onions and carrots. Lay the celery stalks on the counter (with leaves, if desired) and roll a rolling pin over them a few times to release the natural juices. Add to stock. Add the larger half of the garlic head. Crush the mushroom stems by hand before adding to the pot. Add the bay leaves, peppercorns, and thyme, if using.

Cover pot. Bring to a boil and simmer for 45–60 minutes. The longer you simmer it, the more flavorful your stock will be. Stir occasionally to circulate the veggies.

Cool slightly and strain. Pour stock back into pot.

Meanwhile, prepare the soup: In a large frying pan, melt the butter. Crush the smaller half of the garlic head with a rolling pin (to facilitate removing the outer skin and to make the garlic more fragrant), and add to butter. Sauté for a minute or two. Add mushrooms, salt, pepper, and nutmeg. Sauté for seven to eight minutes, stirring often, adding basil leaves midway. The mushrooms should be getting soft and wilted.

Add this mixture to the strained stock. Add the cooking cream and additional butter, if desired. Cook for about five minutes or until combined. Remove from heat. Add parsley and let sit, covered, for a few minutes.

Blend with an immersion blender. Taste and adjust seasoning if necessary.