Begin by placing the dried porcini mushrooms in a bowl and covering them with hot water. Allow them to soak for approximately 20 minutes or until they become soft and plump. Once rehydrated, strain the mushrooms, ensuring to reserve the soaking liquid.

Heat a large pot over medium heat and melt a generous amount of butter. Add finely chopped onions and minced garlic to the pot, sautéing them until they turn translucent and release their fragrant aromas.

Introduce the sliced fresh porcini mushrooms into the pot and cook them until they release their moisture and become tender. This process usually takes around 5 to 7 minutes. Stir occasionally to ensure even cooking and to develop their flavors.

Incorporate the Rehydrated Mushrooms: Chop the rehydrated porcini mushrooms and add them to the pot, stirring them into the mixture. Allow them to cook for a couple of minutes, allowing their intensified flavors to meld with the other ingredients.

Pour in the reserved soaking liquid from the dried mushrooms, along with vegetable or chicken broth. Bring the mixture to a gentle boil and then reduce the heat to a simmer. Let the soup simmer for approximately 20 to 25 minutes.

Once the soup has simmered to perfection, it's time to blend it to your desired consistency. You can use an immersion blender directly in the pot or transfer the soup in batches to a countertop blender. Exercise caution when blending hot liquids to avoid any accidents.

Season and Serve: Taste the soup and season it with salt, pepper, and any additional herbs or spices that suit your palate. Ladle the hot and velvety soup into bowls, and if desired, garnish it with your choice of optional garnishes such as fresh herbs, a drizzle of truffle oil, or crispy croutons.