Arrange the spinach in the bottom of an 11" diameter bamboo or stainless steel steamer. Set the fish on top of the spinach; season with salt and pepper. Cover the steamer, set over simmering water, and steam until the fish is just cooked through, about 5 minutes. Divide the spinach between 2 plates. Set the halibut on the spinach and surround with the cherry tomatoes and olives. Drizzle with the olive oil and lemon juice and serve.