At Water Grill Restaurant in Los Angeles, where fish reigns on the menu, the desserts, created by pastry chef Wonyee Tom, also cause ripples of pleasure. Tom often assembles them in layers of flavors and textures. At the top of one masterpiece I sampled recently, this sophisticated, exceptionally smooth and velvety sour cream sorbet caught my attention. It’s very easy to make at home. To enhance the tang of the sour cream, Tom infuses the sweet syrup base with lemon grass; if it’s not available, pare the peel from three lemons instead and add to the syrup as it boils.