Use a mandoline or hand slicer to slice the apple and fennel wafer thin and toss in a large bowl. Mix with a squeeze of lemon juice to prevent them from turning brown. Use a pairing knife to peel the grapefruit and then cut out each segment. Add the segments to the bowl along with the remaining ingredients.
Dress the salad with the olive oil, lemon and grapefruit juice and mix well. Check for seasoning, you may need a little more lemon juicy to make the salad tangy.