Preheat oven to 350-degrees Fahrenheit.
Place the pork tenderloin in a baking dish that has been sprayed with olive oil spray.
Season the pork with salt and pepper.
Roast in the preheated oven for 40-45 minutes, or until a meat thermometer reaches 140-degrees Fahrenheit.
Meanwhile, melt the butter over medium heat in a large skillet.
Add the onions, olive oil, and mushrooms, and cook, stirring occasionally for approximately 4-5 minutes.
Stir in the flour and cook, stirring occasionally for an additional 1-2 minutes.
Add the beef stock, and increase the heat to medium-high, stirring occasionally as the gravy thickens.
Turn off the heat and add the bourbon. Stir well.
Return the gravy to the heat, stirring well, and heat through.
When the pork has reached an internal temperature of 145-degrees, remove it from the oven and let it sit for 5 minutes before slicing.
Slice the pork and place on a platter.
Spoon the mushroom bourbon gravy over the sliced pork and serve immediately.