Preparation / Directions:
Season the halibut steaks with salt and black pepper.
Halve the avocado and reserve one half, rubbed with the lemon juice.
In a blender puree the remaining avocado half, peeled, the lime juice, 2 tablespoons of the water, the scallion, the cumin, the cayenne, the sour cream, and salt and pepper to taste, scraping down the sides of the blender, until the sauce is smooth.
Transfer the sauce to a small bowl and stir in enough additional water to thin the sauce to the desired consistency.
Grill the halibut, brushed generously on both sides with the oil, in an oiled ridged grill pan or on an oiled rack set about 6 inches over glowing coals for 4 to 5 minutes on each side, or until it just flakes.
Arrange the steaks on 2 plates, nap them with the sauce, and garnish each serving with the reserved avocado, peeled and sliced thin, fanning the slices, and the lime slices.
Sprinkle the plates with the tomato.