Cut the bottom of the oranges so they stand. Then, use a sharp knife to cut the rind from the orange. Place the orange on its side and cut into thin slices; set aside.
To cut the fennel, cut off a bit of the top and root end. Then, slice the bulb in half lengthwise, place the flat end on the cutting board and cut into thin strips crosswise; set aside.
To make the dressing, whisk together the olive oil, red wine vinegar, honey, dijon mustard, and salt and pepper; set aside.
Assemble the salad by tossing half of the dressing with the mixed baby greens and sliced fennel. Arrange the salad greens on a large platter and top with sliced oranges and red onion. Drizzle with remaining dressing and top with chopped pistachios (optional). Serve immediately.
