1Remove crusts from bread and flatten each slice with a rolling pin.
2Clean fennel bulb by trimming the root ends and cutting off the fibrous green stalks. Halve fennel bulb lengthwise, remove any outer layers that are brown or shriveled, and trim away the core. Using a mandoline or a very sharp knife, cut into paper-thin, lengthwise slices. Reserve 3/4 cup sliced fennel for the sandwiches; save leftovers for another use.
3Slice apple in half lengthwise, and remove the core with a melon baller or a paring knife. Use a mandoline or a very sharp knife to slice apple lengthwise into paper-thin pieces. Reserve 1 cup sliced apple for the sandwiches; save leftovers for another use.
4Combine reserved fennel and apple slices, liqueur, and oil in a medium bowl, season generously with salt and freshly ground black pepper, and toss until well coated. Taste and adjust seasoning as necessary.
5Spread 1 tablespoon of cheese on one side of each slice of bread. Drain fennel-and-apple mixture to remove excess marinade, and evenly distribute among 4 slices of bread, laying it over goat cheese. Close the sandwiches with 4 remaining slices of bread. If making tea sandwiches, trim the edges again, and cut each sandwich into 4 triangles; serve immediately.