In a medium saucepan over medium heat, heat the olive oil.
Sauté the onion until slightly translucent, about 5 minutes.
Add the mushrooms and garlic, and cook for another 5 minutes.
Add the Marsala wine, and deglaze the pan, scraping up any browned bits from the bottom, and add the beef broth.
Season with salt and pepper, and cook down until the sauce begins to thicken, 8 to 10 minutes.
Reduce the heat to low, and simmer until ready to serve.
Preheat the oven to 400°F.
In a large ovenproof skillet over high heat, sear the steaks for 2 to 3 minutes per side, until browned.
Transfer to the oven, and cook for 6 to 10 minutes, depending on how rare you want them.
Remove from the oven, and place 1 tablespoon of butter on each steak.
Let rest for 10 minutes before slicing.
Top with the Marsala sauce, and serve.