Place mache, all the baby lettuces, and watercress in a large bowl, and toss to combine. Transfer to a serving platter.
Cut both ends from the grapefruit. Starting at one cut end, slide a paring knife between the peel and the pulp, removing the entire skin and the outer membrane. Use the knife to lift out the sections. Arrange the sections down the middle of the greens in a row. Place the avocado cubes to one side of the grapefruit sections.
Remove the seeds from the papaya, and reserve for vinaigrette. Cut papaya into 1/2-inch-thick slices, and arrange slices on other side of the grapefruit sections. Top with onion rounds. Drizzle the vinaigrette over the entire salad, and serve immediately.