Step 1Combine the olive oil, vinegar, orange juice, zest, honey and salt and pepper to taste in a blender and process for a few seconds, or whisk everything together in a bowl.
Step 1Heat the oven to 350 degrees. Spread the nuts on a baking sheet and toast them until lightly browned, 6 to 8 minutes. Set them aside.
Step 2Watercress can be muddy, so wash it thoroughly, then tear the small sprigs from the branches, discarding the heavy stems. Spin it in a salad spinner to dry. Rinse and tear the butter lettuce and spin dry.
Step 3Peel the tangerines, remove all the white pith and separate the sections. Slice the peel off the grapefruit and separate the sections, removing the membranes. Peel the avocados and cut them into 1-inch pieces. Unless you are cutting the avocados at the last moment, sprinkle a little lemon juice on them and cover them.
Step 4Combine the watercress, lettuce, tangerines, grapefruit, avocado and crumbled cheese in a large salad bowl and toss together with your hands. Add most of the dressing and continue lifting and turning very gently until everything is evenly bathed, then taste and add a bit more dressing only if it's needed. Arrange the salad on individual plates and sprinkle the toasted walnut pieces on top.