Preheat the water bath to 131°F (55°C).
Mix the spices together in a bowl. Salt and pepper the steak and sprinkle with some of the spice mixture. Add to the sous vide bag with the thyme, seal, and place in the water bath. Cook the steak for 2 to 4 hours, until heated through.
For more information on the cooking times you can read my detailed article which addresses why is there a range in sous vide cooking times.
Preheat an oven to 400°F (200°C).
Cut the ends off the Brussels sprouts and discard the ends. Cut the remaining portion in half lengthwise. Toss the Brussels sprouts and garlic in olive oil then salt and pepper them. Place on a roasting sheet then cook, stirring once or twice, until tender, about 30 to 45 minutes.
During the last 5 or 10 minutes, add the cherry tomatoes to the roasting sheet and cook until they just start to burst.
Heat some oil in a pan over medium heat. Add the onions and garlic then cook until the onions become tender and start to lightly brown, about 15 to 20 minutes. Add the white wine vinegar and stir to mix well.
Take the steak out of the water bath and remove it from the bag. Dry it off thoroughly using paper towels or a dish cloth. Quickly sear the filet mignon for 1 to 2 minutes per side, until just browned, then remove from the heat.
Place the roasted Brussels sprouts and tomatoes in a bowl and drizzle with the fresh lemon juice. Add the steak and top with the sautéed onions. Sprinkle with the thyme leaves then serve.