In a dry skillet over medium heat, toast the fennel seeds until fragrant, about 3-4 minutes. Remove from heat and let cool slightly. In a small bowl, combine the toasted fennel seeds, sumac, salt, and pepper.

Pat the filet mignon steaks dry with paper towels. Rub each steak with olive oil on all sides. Press the fennel seed and sumac mixture onto each steak, ensuring an even coating.

Heat a large skillet over medium-high heat. Add the butter. Once melted and bubbling, add the steaks to the skillet. Cook the steaks for about 4-5 minutes on each side for medium-rare, or to your desired doneness. Remove from the skillet and let rest.

In the same skillet, add the onions and garlic, sautéing until translucent. Add the carrots, parsnips, and potatoes. Cook for about 5 minutes, stirring occasionally.

Pour in the vegetable broth and bring to a simmer. Add thyme, balsamic vinegar, salt, and pepper. Cook until the vegetables are tender, about 15-20 minutes.

For a thicker gravy, stir in the cornstarch mixture and simmer until thickened, about 2-3 minutes.

Slice the filet mignon and plate it alongside a generous portion of the velvety root vegetable gravy. Garnish with additional fresh thyme if desired.