Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Prepare the vegetables by halving the Brussels sprouts, slicing the carrots into thin rounds, and cutting the red onion into small wedges.
Place the vegetables on the baking sheet and toss with half of the olive oil, half of the sea salt, half of the black pepper, and all of the dried thyme.
Spread the vegetables in a single layer and roast for 20-25 minutes until they are tender and have developed a golden-brown char.
While the vegetables are roasting, season the chicken breast on both sides with the remaining sea salt and black pepper.
Heat the remaining olive oil in a heavy-bottomed skillet over medium-high heat.
Place the chicken in the skillet and sear for 6-7 minutes per side, or until the internal temperature reaches 165°F.
During the last minute of cooking, add the minced garlic to the pan and finish with a splash of fresh lemon juice to deglaze.
Remove the chicken from the pan and let it rest for 3 minutes before serving alongside the roasted vegetables.