Heat the fish stock to simmering in a small saucepan and leave on very low heat. 

In a heavy-bottomed, wide saucepan heat the olive oil, then add onions and cook until translucent over medium heat. Add the rice and stir to coat each and every grain with oil. Cook until rice is opaque, 1 to 2 minutes. Add the sake and cook, stirring, until it has evaporated. Stir in 1 cup of the hot stock and allow to cook until the liquid is completely absorbed. Continue adding stock in 1/2 cup increments, stirring until evaporated between each addition. Continue until rice begins to soften, with just a trace of stiffness left in the grains of rice, about 16 to 18 minutes. The last addition of stock should just absorb into rice, leaving it slightly tight. Stirring quickly with a wooden spoon, add the butter and cream. The risotto should be very creamy. 

Remove from heat and adjust seasoning to taste. Serve garnished with the green onion tops and freshly-grated lime zest if desired.