Wash the curly endive well, tear it into bite-sized pieces, and spin it in a salad spinner or use kitchen towels to remove excess moisture. Wash and trim the fennel bulbs, cut them in half lengthwise, and slice very thinly. In a large salad bowl, toss together the endive, fennel, green onions, and cilantro leaves.

Halve and peel the avocado, slice it, put the slices in a glass dish or bowl, and immediately pour the lemon juice over them. Drizzle the olive oil over the salad greens, add the avocado and lemon juice and a little salt and pepper, and toss the salad very gently with your hands until everything is well combined and lightly coated with dressing.

With a very sharp knife, slice the peel off the grapefruits, cutting away all the white pith, and then pull them apart into sections, peeling away the membranes. Scatter the grapefruit sections over the salad and serve.