Preheat the oven to 400°F (200°C).
Spread cauliflower, Brussels sprouts, and carrots on a baking sheet. Drizzle with avocado oil, salt, and pepper. Toss well and roast for 30 minutes until golden and tender.
Cook the rice by combining rice and chicken stock in a pot. Bring to a boil, reduce heat, cover, and simmer for 10 minutes. Turn off heat and let it rest.
Season the chicken with thyme, paprika, salt, garlic powder, and pepper. Toss to coat evenly.
Heat avocado oil in a skillet over medium-high heat. Cook chicken pieces for 3–4 minutes per side until fully cooked.
In a bowl, whisk together mayonnaise, Dijon mustard, and honey to make the sauce.
Assemble the honey mustard chicken and rice bowls by dividing rice, vegetables, and chicken evenly into bowls. Drizzle with sauce and serve.