Cut the ends of the grapefruits and then carefully cut away the peel and pith, leaving the grapefruit. Using a paring knife now, carefully cut away the grapefruit sections from the membranes over a bowl, so you catch the juice, too.
Drain the grapefruit juice into a smaller bowl where you will make the dressing. Keep 2 tablespoons for the dressing and drink the rest! Add the vinegar, orange zest, mustard, salt and pepper and stir well. Drizzle the olive oil in and whisk until emulsified.
Mound a heaping handful of arugula on each of the four plates. Then divide the endive chunks, grapefruit sections, red onion and avocado and place on all four plates as well. Drizzle the dressing over each and top with a few more grinds of black pepper. Serve immediately.