Preheat the oven to 375 degrees.

Heat the oil and 1 tablespoon of the butter over medium to medium-high heat in a large braiser, deep saucepan or heavy roasting pan.

Season the trimmed tenderloin all over with salt and pepper, then add to the pan.  Cook for 6 to 7 minutes, turning the tenderloin halfway through, so that it is browned on the bottom and top. Use tongs to transfer it to a plate.  It will not be cooked through.

Increase the heat to medium-high and add the mushrooms to the pan. Cook, stirring, for 2 to 3 minutes, then add the onion. Cook for 2 to 3 minutes, just until the onions start to soften around the edges. Season lightly with salt and pepper.

Return the pork tenderloin to the pan, then place the pan in the oven and roast for about 20 minutes, until the internal temperature of the tenderloins registers 155 degrees.

Use tongs to transfer the tenderloin to a plate; cover loosely with aluminum foil to keep warm (temp will rise while tented).

Return the pan with the mushrooms and onions to the stove over medium-high heat. Cook for 3 to 4 minutes, until the pan juices have almost evaporated. Add the Marsala and broth; cook for 3 to 4 minutes, until the liquid has reduced by half.

Cut the remaining tablespoon of butter into several small pieces and add to the pan in several additions, stirring as the butter melts. Taste and add seasoning as needed. Remove from the heat.

Cut the tenderloin crosswise into slices and divide among individual plates. Top with a portion of the mushroom sauce and a sprinkling of parsley. Serve immediately.