To make the sauce, melt butter in a small pot over medium heat. Add onion and carrot and cook 4-5 minutes. Stir in the flour until well combined. Stir in the chicken stock and simmer until carrots are very tender, about 10 minutes. Puree mixture in blender or food processor until smooth. Return to the pot. Add additional stock if too thick. Stir in the lemon juice, herbs, salt and pepper. Set aside until fish is ready to steam.
To make the fish, cook lettuce leafs in boiling water 20-30 seconds, just until slightly wilted. Remove and cool in ice-cold water. Drain well and pat dry. Place leaves, spread out and outer-side down, on a work surface. Place a halibut fillet in the centre of each leaf. Brush with melted butter and season with salt and pepper. Fold over edges of the lettuce and seal the fish inside. Place fold side down in a steamer. Cover and steam over boiling water 6-8 minutes, or until cooked through. While this occurs, return the sauce to a simmer. When cooked, serve halibut on a generous pool of the carrot sauce.