In a medium skillet, cook the chopped pancetta over moderately high heat, stirring occasionally, until it is crisp, about 8 minutes. Using a slotted spoon, transfer the pancetta to a plate. Pour off all but 1 tablespoon of the fat in the skillet. Add the sliced onion, cover and cook over moderate heat until it is softened, about 5 minutes. Add 2 tablespoons of water to the skillet and cook, stirring occasionally, until the onion is lightly browned, about 5 minutes longer. Stir in the cumin and season the onion with salt and pepper.
In a bowl, toss the cheese with the flour. In a medium saucepan, bring the wine to a simmer over moderate heat. Add the cheese in handfuls and stir constantly until each batch is completely melted before adding more. Cook, stirring vigorously, until creamy, about 2 minutes longer. Stir in the pancetta and half of the onion and season generously with pepper. Float the remaining onion on top of the fondue and serve with warm pretzels, rye bread cubes, cornichons and pickled vegetables.