In a bowl, mix porcinis and 1 cup boiling water; let stand 10 minutes.
Halve leek lengthwise; thinly slice crosswise. Wash and dry leek. Slice 4 white mushrooms. Trim and quarter remaining ones. Add leek, quartered mushrooms, barley, broth and thyme to a slow cooker.
Lift porcinis out of water. Rinse, rubbing to remove grit, and drain; add to slow cooker. Strain soaking liquid into slow cooker; stir. Cover and cook on high until barley is tender, 2 1/2 to 3 hours.
Remove and discard thyme sprigs. Season with salt and pepper. Stir in half of parsley. Divide risotto among bowls. Scatter reserved mushrooms and remaining parsley on top. Serve immediately.