Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Trim the Brussels sprouts and cut them in half, then break the broccoli into uniform, bite-sized florets.
Dice the boneless skinless chicken thighs into 1-inch pieces to ensure they roast quickly and evenly.
Place the chicken, broccoli, and Brussels sprouts on the prepared sheet pan in a single layer.
Mince the garlic cloves and whisk them in a small bowl with the olive oil, sea salt, black pepper, and dried oregano.
Drizzle the garlic-herb oil over the chicken and vegetables, using your hands or tongs to toss everything until thoroughly coated.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with crisp, charred edges.
Remove the pan from the oven and immediately stir in the cooked lentils, lemon juice, and lemon zest to brighten the flavors before serving.