Preheat your oven to 400°F (200°C).
On a large baking sheet, toss the halved Brussels sprouts, sliced carrots, and red onion wedges with 0.5 tbsp of olive oil and half of the salt and thyme.
Spread the vegetables in a single layer and roast for 20-25 minutes until tender and the edges are beautifully browned.
While the vegetables roast, season the chicken breast evenly with the remaining salt, pepper, and thyme.
Heat the remaining 0.5 tbsp of olive oil in a heavy skillet over medium-high heat.
Place the chicken in the skillet and sear for 6-7 minutes per side until the internal temperature reaches 165°F and the exterior is golden.
Add the minced garlic to the pan during the final minute of cooking, tossing it in the pan juices until fragrant.
Remove from heat, drizzle the chicken with fresh lemon juice, and serve immediately alongside the roasted vegetable medley.