Preheat oven to 400°F (200°C). Toss Brussels sprouts with olive oil, garlic powder, onion powder, salt, and pepper. Spread on a baking sheet and roast for 20–25 minutes, tossing halfway, until golden and crispy.
While sprouts roast, mix the cod with olive oil, garlic, lemon juice, oregano, salt, and pepper in a bowl.
Heat a skillet over medium-high heat. Add cod fillets and cook for 3–4 minutes per side until golden brown and flaky. Set aside.
In a small saucepan, melt the butter over medium heat. Add minced garlic and sauté for 1 minute. Stir in lemon juice, parsley, salt, and pepper.
Assemble bowls by dividing roasted Brussels sprouts evenly. Top with cod fillets and drizzle generously with lemon garlic butter sauce.
Serve with extra lemon wedges or your favorite grain like rice or quinoa.