Savory porcini and cremini mushrooms are accompanied by mellow diced Vidalia and yellow onions and shallot in this wintry soup. Before added to the vegetables and beef stock, orzo is toasted in a skillet, giving the pasta a nutty flavor.
Place porcini in a bowl. Cover with 2 cups boiling water; let sit until softened, about 20 minutes. Remove from liquid. Finely chop, and set aside. Strain liquid through fine cheesecloth or a coffee filter, discarding grit; set aside.
Place orzo in a large skillet over medium-low heat. Toast, stirring continuously, until light golden, about 6 minutes. Remove from heat; set aside.
Bring a pot of water to a boil; add toasted orzo; cook until al dente, 3 to 4 minutes. Drain, and set aside.
Warm olive oil in a stockpot over low heat. Add Vidalia onion, yellow onion, shallot, and thyme; cook, stirring occasionally, until onions are very soft and golden brown, 45 to 50 minutes. Increase heat to medium; add cremini mushrooms; saute, stirring frequently, until mushrooms have softened and browned, about 8 minutes. Add Madeira; cook 1 minute more. Add reserved porcini, reserved strained liquid, beef stock, salt, and pepper; bring to a simmer. Add reserved pasta; continue to cook until pasta is heated through, about 2 minutes more. Serve immediately, garnished with chopped parsley.