First, prepare the dressing. To do this, mix the oil and lemon juice with salt and pepper in a mixing bowl. The bowl should also be large enough to comfortably hold the fennel and green apples. So choose accordingly.
Wash the fennel, cut off the root end and the top of the stalk. Slice the bulb into very thin slices and immediately place in the lemon and oil dressing. Leave for 5-10 minutes.
Thinly slice the apple and add it to the dressing. Apples and fennel brown quickly when exposed to air, so they need to be added to the lemon and oil mixture quickly.
Toast the walnuts in a pan without oil, then chop them up.
When serving, place the apple slices and sweet potato slices in a deep plate with the sauce and sprinkle the toasted walnuts on top.