Heat olive oil and 3 tablespoons butter in a large pot over medium heat. Add diced onions and shallots. Season gently with salt and cook for about 8–10 minutes until softened and translucent.

Add the remaining butter and the baby portobello mushrooms. Cook for 10–12 minutes until browned and fragrant. Stir in minced garlic and let the aroma fill your kitchen for a minute.

Deglaze the pot with your chosen broth, scraping up the brown bits. Then, add balsamic vinegar, sage, bay leaf, and rosemary. Let this simmer for about 5 minutes.

Pour in the stock and bring to a gentle boil. Reduce the heat and let it simmer uncovered for about 25–30 minutes. Remove the bay leaf and rosemary stems before blending.

Blend until smooth using an immersion blender. Stir in the heavy cream and Parmesan over low heat. Adjust seasoning with salt and pepper.

In a separate skillet, sauté the sliced shiitake mushrooms in 1 tablespoon of olive oil for about 5–6 minutes and set aside.

Toast the crusty bread cubes in the skillet with the remaining olive oil and chopped fresh rosemary until golden and crisp.

Ladle the soup into bowls. Top with sautéed shiitake mushrooms, crispy rosemary croutons, and an extra sprinkle of freshly grated Parmesan.