Put the dried mushrooms in a large glass measuring cup and pour 2 cups of boiling water over them. Set aside while you chop the vegetables.
Heat the olive oil in a large soup pot over medium-low heat. When it shimmers add the onion, garlic, and carrot, and cook, stirring occasionally, until the vegetables soften, 8 to 10 minutes. Add the zucchini and cook another 3 minutes. Add the fresh mushrooms and cook another 3 minutes.
Add the barley and sprinkle with salt & pepper. Stir to combine and cook another minute or two.
While that’s going, pluck the porcini from the hot water and chop finely, then add to the pot. Pour in the mushroom soaking liquid—but be careful not to add the gritty sediment at the bottom. Add the broth and the bay leaf and bring to a boil. Reduce heat, cover, and simmer for 25 minutes.
When the timer goes off, add the thyme sprig and simmer another 15 minutes, until the barley is soft but not mushy. Remove the bay leaf and thyme stems before serving.