Position racks in upper and lower thirds of oven; preheat to 400°F.
Combine Brussels sprouts, fennel, 1 Tbsp. oil, and ¼ tsp. each salt and pepper in a large bowl; toss to coat. Spread in a single layer on a large rimmed baking sheet. Bake, stirring occasionally, until tender, 20 to 25 minutes.
Meanwhile, place halibut on another large rimmed baking sheet and top with half of the garlic and the remaining ¼ tsp. each salt and pepper. Combine lemon juice and melted butter in a small bowl. Drizzle or brush half of the mixture over the fish. Bake until the fish is opaque and flakes easily with a fork, 12 to 15 minutes.
Meanwhile, combine quinoa, the remaining 1 tsp. oil, sun-dried tomatoes, olives, and parsley (or fennel fronds) in a medium bowl.
Add the remaining garlic to the lemon-butter mixture. Pour the mixture over the vegetables and bake for 1 minute more. Serve the halibut and vegetables alongside the quinoa mixture.