Place the dried mushrooms in a bowl and cover with 2 cups of warm water. Let the mushrooms soak at room temperature for 30 minutes to soften.
Meanwhile, chop the fresh mushrooms.
Melt the butter in a medium-sized saucepan over medium heat. Add the onions, fennel, carrots and garlic. Season with salt and pepper. Sauté, stirring occasionally, until the vegetables are pretty soft, and very lightly browned, about 5 minutes. Add the thyme and fresh chopped mushrooms and cook, stirring frequently, until they release their liquid, and then the liquid evaporates and the mushrooms caramelize a bit. Stir in the flour and cook, stirring very frequently, for 3 minutes, until the flour lightly browns and coats the vegetables.
Add the chicken broth and bay leaves and increase the heat to high. When the soup is simmering, drain the soaking mushrooms, reserving the liquid. Strain the liquid into the soup through a fine mesh strainer (if you want to go the extra mile and line the strainer with cheesecloth to make sure you remove every bit of possible dirt, go for it, but it’s not necessary unless your liquid looks very dirty). Rinse the soaked mushrooms and cut off and discard any parts that are hard or gritty. Chop the mushrooms coarsely and add them to the soup with the spelt. Continue to simmer, uncovered, until the spelt is tender, 60 to 90 minutes, stirring from time to time. Add more broth or water as needed to keep the consistency you want. Check the seasonings towards the end and add salt and pepper if needed.
Remove and toss the bay leaves. Ladle into bowls, top with parsley and pickled onions (if using) and serve hot.