Wash and dry all produce. Preheat oven to 400 degrees. Trim and halve Brussels sprouts lengthwise. Peel and cut carrot and parsnip into 1/4-inch slices on a diagonal. Strip thyme off stems and discard stems. Halve lemon. Bring 1 cup water and a pinch of salt to a boil in a small pot.

Toss Brussels sprouts, carrot, and parsnip on a baking sheet with half the thyme, a drizzle of olive oil, and a pinch of salt and pepper. Roast until lightly browned, 20-25 minutes. Toss halfway through.

Add rice to boiling water, cover, and reduce to a simmer. Cook until tender, 15-20 minutes.

Heat a drizzle of olive oil in a large pan over medium-high heat. Season chicken all over with salt and pepper. Add to pan and cook until browned, 2-3 minutes per side. Transfer chicken to a baking sheet and roast until juices run clear when pierced with a knife, 5-10 minutes. Set aside to rest 5 minutes.

Heat a drizzle of olive oil in same pan over medium heat. Stir in chicken demi-glace, remaining thyme, and 1/2 cup water. Simmer until thickened and reduced by half, 2-3 minutes. Remove pan from heat and stir in a squeeze of lemon and 1/2 TBSP butter. Season with salt and pepper.

Thinly slice chicken. Fluff rice with a fork and toss with 1/2 TBSP butter. Serve chicken on a bed of rice with veggies. Drizzle with pan sauce.