In a medium bowl, whisk together the spicy mustard with lemon juice, salt, pepper, and thyme. Add the chicken breasts and toss to coat. Cover and transfer to the refrigerator to marinate for 15 to 30 minutes.
Preheat oven to 350 F. Prepare a baking sheet with parchment paper.
Toss the Brussels sprouts in a medium bowl with melted coconut oil, and a little more salt and pepper. Remove the chicken from the fridge and let it sit for 15 minutes at room temperature.
Transfer the Brussels sprouts to the prepared baking sheet in an even layer, and transfer the chicken to a glass baking pan.
Bake the chicken at 375 F for 10 minutes. After 10 minutes, add the Brussels sprouts to the oven and roast for 15 minutes until softened and a bit crispy. (You can always remove the chicken at this point and let the sprouts go a little longer to crisp them up.)
Remove from the oven and divide the meal into two servings, placing in individual meal prep containers and transfer to the refrigerator.