Combine mushrooms and 2 cups boiling water in a bowl; cover and let stand 15 minutes. Drain mushrooms in a colander over a bowl, reserving liquid. Chop mushrooms; set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add onion, garlic, and mushrooms to pan; sauté 5 minutes or until tender. Add the reserved mushroom liquid, spinach, and next 5 ingredients (through broth); bring to a boil. Cover, reduce heat, and simmer 10 minutes. Garnish with thyme and red pepper, if desired.