Position a rack in the center of the oven and heat the oven to 250 degrees F.

In a small bowl, combine the olive oil, thyme, garlic, 1 teaspoon salt, and several generous grinds of black pepper. Put the tenderloin on a heavy-duty rimmed baking sheet or in a small roasting pan and rub the oil mixture all over it.

Roast the tenderloin until an instant-read thermometer inserted in the thickest part reads 130 degrees F for medium rare, about 1 hour. Transfer to a carving board, tent with foil and let rest for at least 20 minutes before serving.

Cut the tenderloin crosswise into 1/2-inch slices. Serve with the ragout.

Soak the porcini in 1-1/2 cups very hot water, stirring occasionally, until they're rehydrated, about 20 minutes. With a slotted spoon, transfer them to a cutting board and chop coarsely. Strain the soaking liquid through a coffee filter into a small bowl and set aside.

In a 10-inch straight-sided saute pan, heat 2 tablespoons of the butter with the olive oil over medium heat. Add the cremini and 1 teaspoon salt and cook, stirring occasionally, until the mushrooms have softened and released their liquid, 5 to 8 minutes. Increase the heat to medium high and cook, stirring more frequently, until the mushrooms are shrunken and very well browned, 8 to 10 minutes more.

Reduce the heat to medium, add the shallots and the remaining 1 tablespoon butter and cook, stirring, until the shallots are softened, 1 to 2 minutes. Add the Marsala, thyme, porcini, and 1/4 cup porcini-soaking liquid (reserve the remaining soaking liquid if making ahead). Cook and stir until most of the liquid evaporates, 1 to 2 minutes. Add the cream and cook until reduced to a saucy consistency, 1 to 2 minutes. Stir in the parsley and season to taste with salt and pepper.