Brush peanut oil over both sides of fish. Stuff lemon grass, gingerroot, and lime slices into fish cavity.
Add water to fish poacher or large, shallow roasting pan to a depth of 1 inch. Place steaming rack in pan; place pan over 2 burners, and bring water to a boil. Arrange fish on rack; cover, reduce heat, and simmer 18 minutes or until fish flakes easily when tested with a fork. Remove fish to a serving platter.
Combine mint and next 5 ingredients in a bowl, and stir well. Spoon sauce over fish; garnish with mint, if desired.