Place porcini mushrooms in a small bowl; cover with 1 cup boiling water. Let stand 20 minutes or until soft. Drain mushrooms in a sieve over a bowl, reserving liquid.
Place reserved mushroom liquid, 1 1/2 cups water, rice, and 1/4 teaspoon salt in a large saucepan; bring to a boil. Cover, reduce heat, and simmer 50 minutes or until rice is tender.
While rice cooks, heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add onion; sauté 2 minutes. Add remaining 2 teaspoons oil, 1/8 teaspoon salt, portobello mushrooms, and button mushrooms; cook 5 minutes or until mushrooms are tender and liquid mostly evaporates, stirring frequently. Stir in remaining 1/8 teaspoon salt, porcini mushrooms, and chopped thyme.
Remove rice from heat; stir in mushroom mixture, cheese, and pepper.
Coat 4 timbales or 4 (6-ounce) ramekins with cooking spray. Pack about 1 cup rice mixture into each timbale; invert onto plates. Garnish with thyme, if desired.