Heat 3 tablespoons of olive oil and 3 tablespoons of butter in a large pot over medium heat. Add the diced onions and shallots, season with salt and pepper, and sauté for about 8–10 minutes until softened and translucent.

Add the remaining butter and the quartered baby portobello mushrooms. Sauté for 10–12 minutes until they’re nicely browned.

Stir in the minced garlic and cook for an additional minute.

Deglaze the pot with broth, scraping up any flavorful brown bits. Toss in balsamic vinegar, sage leaves, bay leaf, and rosemary. Simmer for about 5 minutes.

Pour in the stock, bring to a gentle boil, and then reduce the heat. Simmer uncovered for 25–30 minutes.

Remove the bay leaf and herb stems before blending.

Using an immersion blender, carefully blend the soup until it’s smooth and velvety. Return it to low heat, and stir in heavy cream and freshly grated Parmesan. Taste and adjust seasoning as needed.

Sauté the shiitake mushrooms in 1 tablespoon of olive oil for 5–6 minutes, seasoning as desired. Set aside.

For the croutons, toast the cubed bread in the remaining olive oil until golden and crisp, adding in chopped fresh rosemary.

Ladle the soup into bowls, topped with sautéed shiitakes, rosemary croutons, and a sprinkle of freshly grated Parmesan.