Peel and supreme (segment) oranges over a bowl to catch the juice. Squeeze what is left of each orange to extract all the juice. Remove any seeds. Carefully strain juice from segments and reserve for dressing.
Remove fronds from fennel bulb if necessary-you only want to use the bulb part for this as the stem is tough-but save it for making stock. Thinly slice fennel and onion-I use a mandolin for this to create uniform very thin slices but if you don't have one a sharp knife will do the job just fine.
Wash and dry lettuce-leave whole leaves intact if they are small enough or gently rip into good-sized pieces. I was lucky enough to find fresh locally greenhouse-grown baby sized lettuce from a local farm this time of year-score!
Make Dressing: Place all dressing ingredients into a bowl and whisk until emulsified. Then place sliced fennel and onion into the dressing to marinate for ½ hour up to 1 hour.
Assemble Salad: Portion out and lay lettuce on plates. Spoon fennel/onion mixture and more dressing if desired on top of lettuce. Arrange orange segments over salad.
Fennel and onion can marinate in dressing overnight in the refrigerator. You can use any lettuce of your liking or whatever you have on hand. For a video tutorial on how to supreme an orange click here. Toasted pine-nuts would make a nice addition sprinkled on top.