1) Make the dressing by combining orange juice, vinegar, and mustard in a bowl. Slowly drizzle in oil, whisking constantly. Season with salt and pepper.
1) Prepare the salad by cutting the tops and bottoms off of the oranges to make a stable cutting surface. Place oranges flat on a cutting board, and using a sharp knife slice, down on the sides to remove the peel and white pith. Dice the whole orange.
2) Mix lettuce, red onion (optional), orange chunks and feta cheese. Just prior to serving, toss lightly with dressing and toasted almonds.