1. Place a large sauté pan on medium-high heat and let it get hot, about 45 seconds. Add the olive oil, mushrooms, and bell pepper, and sauté, stirring often, until they are almost tender, about 4 minutes. Add the shallots and sauté 3 minutes more, stirring frequently.

2. Transfer the vegetables to a medium bowl and return the pan to the stove. Increase the heat to high and add the butter. As soon as the butter stops foaming, add the slices of tenderloin.

3. Brown the meat quickly, about 2 minutes on each side, then transfer to a plate.

4. Add the white wine, Marsala, and bouillon cube to the pan and reduce until the liquid is almost entirely evaporated, about 3 minutes.

5. Return the vegetables and meat to the pan. Cook just long enough to reheat the meat and vegetables, 1-2 minutes, turning the meat a few times. Season with salt and pepper, if desired. Serve immediately, pouring the pan juices over the meat. Garnish with parsley.