In a small bowl, cover the porcini with 1 cup warm water and let soften 30 minutes. Lift the porcini out with a slotted spoon and chop fine. Strain the liquid through a double thickness of damp paper towels.

Bring a large pot of salted water to a boil for the pasta.

Heat the oil in a large skillet over moderate heat. Add the onions and sauté until softened, about 5 minutes; don't let them color. Add the garlic, parsley and thyme and cook them for a minute or two to release their fragrance.

Raise the heat to moderately high and add the chicken. Cook until the meat loses its pinkness, breaking it up with a wooden spoon. Season with salt and pepper. Add the wine and simmer until it evaporates. Add the chicken broth, 1/2 cup porcini liquid and the porcini. Simmer for a couple of minutes to reduce the liquid, then stir in the marinara sauce. Reduce the heat to maintain a gentle simmer and cook until the sauce is well-flavored and slightly thickened, about 5 minutes. Stir in the butter. Keep warm over low heat.

Boil the pasta until it is about 1 minute shy of al dente. Set aside 1 cup of the pasta water, then drain the pasta. Return the pasta to the warm pot, add the sauce and place over moderate heat. Cook the pasta for about a minute so it absorbs some of the sauce. Add 1/4 cup of the cheese and toss again, adding some of the reserved pasta water if the sauce appears dry. Divide the pasta among warm bowls and pass more Parmesan at the table.