Preheat oven to 400°F. Lightly grease a 9x13 baking dish.
Cook garlic in olive oil and butter over medium heat 1 to 2 minutes until fragrant. Do not brown.
Add coconut milk, lemon juice, lemon zest, onion powder, turmeric, paprika, salt, pepper, and red pepper flakes. Simmer 3 to 4 minutes until slightly reduced. Taste and adjust seasoning.
Pat cod fillets dry, season lightly with salt and pepper, fold thin tail ends under, and arrange in the baking dish.
Pour coconut lemon sauce over and around fillets. Place lemon slices on top of each fillet.
Bake uncovered at 400°F for 16 to 20 minutes, spooning sauce over fillets once during the last 5 minutes. Fish is done when it flakes easily and is fully opaque. Rest 2 minutes, garnish with fresh herbs, and serve.