OPTIONAL: Start the onions by heating the olive oil over medium low heat. Add the chopped onion and cook for 40 minutes, until the onions are caramelized and tender.
In a large skillet over high heat, melt the butter until frothy and beginning to brown. Season the filets with salt and pepper, to taste.
Place in the hot skillet and cook, 4-5 minutes per side, or until the internal temperature reaches 120°F (rare) -130°F (medium rare). Lower the heat and add the truffle butter. Swirl the pan to mix the butters. Move the steaks to one side and tilt the pan so that the butters cover the steaks, or use a spoon to ladle the butter over the steaks so that a golden brown crust forms.
Serve the filet mignon topped with caramelized onions and a bed of simple garlicky mashed potatoes. Garnish with parsley, as desired. Enjoy!