How to Make It
In a small saucepan, bring the white wine vinegar to a boil. Remove from the heat. Add the chopped jalapeños and 1/2 teaspoon of salt and let the jalapeños stand for 2 hours. Strain, reserving the jalapeños and vinegar separately.
In a large enameled cast-iron casserole or Dutch oven, cook the bacon over moderate heat until browned and crisp, about 6 minutes. Using a slotted spoon, transfer the bacon to a plate.
Add the shallots to the casserole and cook over moderate heat until softened, about 3 minutes. Add 1/2 cup of the stock and bring to a boil. Add the chard stems, cover and cook until softened, about 4 minutes. Add the chard leaves and the remaining 1/2 cup of stock and season with salt and pepper. Cover and cook, stirring occasionally, until the leaves are tender, about 5 minutes. Stir in the crispy bacon and 2 tablespoons of the jalapeño vinegar and transfer to a bowl. Serve the pickled jalapeños and the remaining jalapeño vinegar on the side.