Using a knife halve the avocado, remove the pit, then slice. Place the slices in a small bowl. Sprinkle with 1 tablespoon of the lemon juice and toss gently with a rubber spatula to coat. Add the fennel, the onions and the remaining 1 tablespoon of lemon juice and gently toss together.
Place the arugula leaves, cilantro, olive oil, salt and pepper in a large bowl. Add the avocados and fennel and toss together. Serve immediately.