Lettuce greens, torn into bite-size pieces, as needed
To pickle the onions:
Cut the onion in half. Slice thinly, about 1/8-inch thick. Place in a bowl and pour boiling water to cover. Let sit 2 minutes; then drain off the water.
In a saucepan, heat the white wine vinegar, honey, bay leaf, sliced ginger, red pepper flakes, mustard seeds and thyme, if using.
Pour over the drained onion in a stainless steel or glass bowl. Cover and refrigerate, at least one hour.
Assemble the salad:
Slice the fennel as thinly as possible, using a mandolin or a sharp knife.
Peel the orange. Holding over a bowl, remove the segments. Add the orange segments to the fennel, saving the juice. Whisk together the orange juice, the vinegar, olive oil, salt and cayenne pepper to make a vinaigrette.
Sample the pickled onions and add all or a portion to the fennel and oranges.
Dress the fennel, orange and onions with the vinaigrette. Arrange the lettuce on plates. Divide the fennel mixture between the plates and drizzle with the remaining vinaigrette.
Peel ginger by scraping with the back of a spoon—a spoon gets into the crevices between the branching knobs much easier than a paring knife.