1. Fill a large bowl with ice water. In a large pot of boiling water, cook the corn until the kernels become slightly translucent, about 3 minutes. Transfer the corn to the ice water; let cool. Drain the corn and pat dry. Cut off the kernels.
2. Roast the bell peppers and jalapeños over a gas flame, turning, until charred all over, about 5 minutes. Transfer to a bowl, cover and let cool slightly. Peel, stem and seed the peppers and jalapeños. Cut the bell peppers into 1/2-inch dice and the jalapeños into 1/4-inch dice.
3. In an enameled cast-iron casserole, combine the vinegar and sugar and bring to a boil, stirring. Add the corn, peppers, jalapeños, onion and mustard and bring to a boil. Simmer for 15 minutes. Season with salt and let cool.
4. Ladle the corn relish into four 1-pint canning jars and refrigerate for up to 3 months.