Make the marinade by combining 2 tbsp soy sauce, juice of ½ lemon, 1 tbsp brown sugar, and a few pepperoncini flakes in a bowl.
Add the 4 boneless chicken thighs to the marinade, toss to coat, and refrigerate for about 30 minutes.
Rinse 300 g brown rice, then cook it in lightly salted water according to package instructions until tender; set aside.
Preheat a large frying pan or skillet over high heat with a little cooking oil and sear the marinated chicken thighs on both sides until browned, about 2–3 minutes per side.
Transfer the seared chicken to an oven-safe dish and bake at 180°C (356°F) for about 25 minutes, or until cooked through.
While the chicken cooks, heat 20 ml cooking oil in the same or a clean pan over medium-high heat, then sauté the julienned carrot, parsnip, and sliced ½ onion until tender-crisp, about 4–6 minutes.
Add the cooked brown rice to the vegetables, stir in 1 tsp soy sauce, and toss until evenly combined and heated through.
Serve the baked chicken thighs on top of or alongside the rice and vegetable stir-fry and garnish as desired.
