1. Preheat oven to 400F.
2. Place basa fillet on a large baking pan, and drizzle with one table spoon olive oil. Add one clove crushed garlic, rosemary and lemon slices for garnish, and salt and pepper to taste.
3. Place fish in oven and cook for roughly 35 minutes. The fish will be opaque and flaky when it's done. If cooking from fresh, cook for about 15 minutes.
4. While the fish is in the oven, heat remaining olive oil in a saucepan over medium heat. Add in butter, and while it's melting, add the remaining garlic, lemon juice, parsley, rosemary, and salt and pepper. Once butter is melted and garlic is a light golden brown, turn to low heat and allow the sauce to sit until the fish is done.
5. Serve fish on a bed of greens or with your favourite roasted vegetables. Drizzle sauce over fish and vegetables and enjoy! Garnish with lemon slices and rosemary.