Cut the avocados in half. Gently twist to separate the halves. Remove the pit. Cut in quarters and remove the peel. Slice into ¼ inch slices. Squeeze the lemon juice over the avocado slices.
Combine the olive oil, mustard, white balsamic and a generous pinch of sea salt in a glass jar with a tightly fitting lid. Shake well.
Arrange avocado, oranges, tomatoes and red onion on a large platter or individual salad plates. Drizzle with the dressing. Shower with fresh herbs and a few generous grinds of black pepper.