In a small bowl, stir together the melted butter, garlic, ¾ teaspoon of the salt, ½ teaspoon of the pepper, cumin, paprika, coriander, and cayenne. Brush the mixture over the tops of the chicken thighs.
In a large bowl, combine the cauliflower, Brussels sprouts, and bacon. Drizzle with the avocado oil and sprinkle with the remaining ¾ teaspoon of salt and ½ teaspoon of pepper. Toss to coat well.
Spread the vegetables out in a rimmed baking sheet. Place the chicken thighs on top. Bake 35 to 40 minutes, until the center of the chicken thighs registers 165°F on an instant read thermometer.
Turn on the broiler and set the pan on the second highest rack of the oven. Broil 2 to 4 minutes to brown the chicken.