In a large heavy-bottomed Dutch oven or stockpot, melt 2 tablespoons of butter over medium-high heat. Add sliced cremini and shiitake mushrooms in a single layer and cook undisturbed for 4 minutes until deeply browned and caramelized. Season with a pinch of salt and transfer half the sautéed mushrooms to a small plate for garnish.

Add the remaining 1 tablespoon butter to the Dutch oven. Toss in the diced onion, carrots, and celery (mirepoix). Sauté for 5 to 6 minutes until the vegetables soften and become fragrant.

Stir in minced garlic, fresh thyme, and rosemary, cooking for 60 seconds until aromatic. Sprinkle the flour over the vegetables and stir constantly for 1 to 2 minutes to cook out the raw flour taste.

Slowly pour in 1 cup of low-sodium broth, whisking continuously to deglaze the browned bits (fond) from the bottom of the pot. Pour in the remaining 4 cups of broth and bring to a gentle boil.

Stir in the rinsed wild rice blend. Reduce heat to low, cover with a tight-fitting lid, and simmer gently for 30 to 35 minutes until the wild rice grains are tender and slightly burst.

Remove the lid and stir in the heavy cream along with the reserved sautéed mushrooms. Simmer uncovered for another 5 minutes until the soup thickens into a velvety, creamy consistency.

Season with kosher salt and freshly cracked black pepper to taste. Ladle hot soup into warm ceramic bowls and serve with thick artisan crusty sourdough bread.