Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Trim the ends of the Brussels sprouts and cut them in half; thinly slice the red onion into wedges.
Place the sprouts and onions on the baking sheet and toss with 0.5 tablespoons of olive oil, half of the sea salt, and half of the black pepper.
Spread the vegetables in a single layer and roast for 20-25 minutes, flipping halfway through, until the sprouts are tender and the edges are beautifully charred.
While the vegetables roast, season the chicken breast on both sides with the remaining salt, pepper, and fresh thyme.
Heat the remaining 0.5 tablespoons of olive oil in a medium skillet over medium-high heat. Sear the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
In a small saucepan over low heat, whisk together the balsamic vinegar and honey. Simmer for 3-5 minutes until the mixture reduces slightly and becomes a thick, glossy syrup.
Remove the chicken from the pan and let it rest for 3 minutes before slicing.
Plate the roasted vegetables alongside the chicken and drizzle the warm balsamic glaze over the entire dish before serving.