Lift rehydrated porcini from liquid, squeeze dry, and chop coarse. Strain rehydrating liquid through sieve lined with paper towel and reserve (should be approximately 3/4 cup).
Add enough chicken stock to mushroom liquid to equal 2 cups. Bring liquid to boil in medium saucepan, add rice blend, and return to boil. Reduce heat to low, cover, and simmer until rice is fully cooked, 40 to 45 minutes. Turn rice into a medium microwave-safe bowl; fluff with fork.
Meanwhile, heat butter in medium skillet over medium heat. Add onions, carrots, and celery; sauté until softened, 3 to 4 minutes. Add shiitake mushrooms; sauté until tender and liquid evaporates. Add porcini mushrooms and thyme; cook, stirring until well coated and blended with other ingredients, 1 to 2 minutes longer. Add this mixture to rice; toss to combine. Add parsley and season to taste with salt and ground black pepper.
Makes: (about 3 cups, enough to stuff six Cornish hens. Servings: