First, make the vinaigrette. In a small jar with a tight-fitting lid, combine the olive oil, fresh orange juice, apple cider vinegar, honey, Dijon mustard, salt, and pepper. Shake well until the dressing is smooth and emulsified. Set aside.
If your walnuts are raw, toast them in a dry skillet over medium heat for 3-5 minutes, stirring often, until fragrant. Remove from the skillet immediately to cool.
Prepare the oranges. Slice the top and bottom off each orange. Stand one orange on a cutting board and slice downwards, following the curve of the fruit, to remove the peel and white pith. Hold the peeled orange over a bowl and carefully cut between the membranes to release the segments.
Thinly slice the red onion and dice the avocado.
In a large salad bowl, combine the mixed greens, orange segments, pomegranate seeds, toasted walnuts, and sliced red onion.
Just before serving, add the diced avocado and crumbled feta to the bowl.
Drizzle about half of the vinaigrette over the salad and toss gently to coat. Add more dressing as needed. Serve immediately.