Using a mandoline, thinly slice the fennel bulb, discarding green fronds. Slice the unpeeled apple on the mandoline, leaving a large inner core (about 1/3 of the original apple). Remove the core and cut the remaining apple into small cubes. Place fennel slices, apple slices and apple cubes in a large bowl. Separately, whisk the olive oil and lemon juice until emulsified, season to taste with salt. To serve, toss the fennel mixture with the dressing and Manchego then top with walnuts. 4 servings.