10 ounces fresh goat cheese, at room temperature (1 1/4 cups)
Freshly ground pepper
20 slices walnut-raisin bread from a rectangular loaf
Preheat the oven to 375°. Spread the pancetta on a baking sheet and bake for 7 minutes, or until slightly crisp and browned. Let cool.
Meanwhile, in a medium saucepan, heat the oil. Add the onion and cook over moderately high heat, stirring, until golden, about 4 minutes. Add the dates and balsamic and sherry vinegars and cook over low heat until the dates are softened and the liquid is syrupy, about 4 minutes. Transfer to a bowl and let cool. Stir in the goat cheese and season with a generous pinch of pepper.
Spread a slightly rounded tablespoon of the goat cheesedate mixture on one side of each slice of bread. Press the pancetta onto half of the slices. Close the sandwiches and cut into quarters.
The sandwiches can be covered with a dish towel and kept at room temperature for up to 2 hours.