Season your filet mignon steaks with 1 tsp of salt and 1 tsp of pepper, coating both sides evenly to enhance natural flavors.
Place the seasoned steaks into a vacuum-seal bag or resealable bag. Add 1 tbsp olive oil and minced garlic to the bag, then seal tightly, removing air by vacuum or water displacement method.
Set your sous vide cooker to 129°F (54°C) for medium-rare. Submerge the sealed bag with steaks into the water bath and cook for 1.5 to 2 hours for tender results.
While steaks cook, combine Greek yogurt, diced cucumber, lemon juice, dill, 1 tsp salt, and 1 tsp pepper in a mixing bowl. Stir well and refrigerate until serving.
Remove steaks from the bag and pat dry with paper towels. Heat a skillet over medium-high heat, add 1 tbsp butter, and sear steaks about 1 minute per side until golden crust forms.
Let steaks rest a few minutes. Serve with a generous dollop of tzatziki sauce and garnish with chopped parsley. Enjoy your tender, juicy filet mignon!