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 Tbs. of the butter with the olive oil over medium heat. Add the cremini and 1 tsp. 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 Tbs. 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.
Make Ahead Tips
You can make and refrigerate the ragoût up to 2 days ahead. Just before serving, reheat it in a medium saucepan over medium heat. Stir in 1 or 2 Tbs. of the reserved mushroom-soaking liquid and 1 or 2 Tbs. heavy cream, letting both reduce slightly until the ragoût is just loose and saucy enough to spoon around the tenderloin. Stir in the parsley.
Photo: Scott Phillips