This curry-like dish, which uses lightly crushed mustard seeds to amp up the flavor of briny-sweet shrimp, is a riff on recipe from “Madhur Jaffrey’s Step-by-Step Cooking.” The best way to lightly crush the seeds is with a mortar and pestle; alternatively, pulse them 3 to 5 times in an electric spice grinder, but don’t overdo it. It’s fine if some seeds remain unbroken. Though chilies plus mustard seeds sounds like a potent combination, the flavors are nicely balanced. However, if you like chili heat, you may wish to leave some or all of the seeds in the serranos when mincing. Serve with plenty of steamed basmati rice.