This dish is prepared much the same way as risotto, but with pasta instead of rice. The mushroom soaking liquid serves as a rich, flavor-enhancing broth.
Place mushrooms in a medium bowl; add 2 cups warm water. Let soak until softened, about 30 minutes. Drain in a sieve lined with a paper towel or coffee filter, reserving liquid. Squeeze mushrooms to remove excess liquid; roughly chop.
Melt butter in a medium saucepan over medium heat. Add onion; cook, stirring occasionally, until softened, 5 minutes. Season generously with salt and pepper. Add orzo; cook, stirring often, until orzo is golden brown and fragrant, about 5 minutes.
Add porcini, reserved liquid, and 2 1/2 cups water. Bring to a boil; reduce heat, and simmer, stirring often, until orzo has absorbed most liquid and is creamy, 15 to 20 minutes. Stir in Parmesan; season with salt and pepper. Sprinkle parsley on top.
Although the name means barley in Italian, orzo is actually made from semolina flour, just like other pastas.