Place dried porcini mushrooms in a medium bowl and pour 1 ½ cups of boiling water over them. Let soak for about 15 minutes until mostly softened.

While the mushrooms soak, dice the yellow onion and mince the garlic cloves. In a Dutch oven melt the butter over medium heat. Add the onion, garlic, and sliced cremini mushrooms. Season with salt and pepper. Cook, stirring occasionally, until the mushrooms are softened and release their juices, about 10 to 12 minutes.

Line a fine-mesh strainer with cheesecloth and pour the soaked mushrooms through it into a measuring cup. Reserve the liquid and coarsely chop the mushrooms, leaving any grit behind in the cheesecloth.

Sprinkle flour over the vegetables in the pot and stir to coat evenly. Cook, stirring regularly, for about 2 minutes until lightly browned.

Pour in the broth scraping the bottom of the pot to release any browned bits. Add the reserved soaking liquid, chopped porcini mushrooms, soy sauce, dried thyme, and bay leaves. Bring to a boil, then reduce the heat to low and let it simmer uncovered for about 30 minutes to develop flavor and slightly thicken.

Remove and discard the bay leaves. Transfer about 2 cups of the soup to a blender or food processor and blend until smooth. Stir the purée back into the pot.

Take the pot off the heat and stir in the heavy cream until combined. Garnish with a sprig of thyme and enjoy!