Finely grate zest from 1 orange into a medium bowl. Then, using a serrated knife, trim off the top and bottom from 2 oranges. Cut away the skin from one orange at a time, completely peeling the orange of its peel and pith (the white stringy skin connecting the peel to the orange). This is called supreming the orange.
Cut each orange horizontally into ¼” rounds. Squeeze enough rounds in your hands for 2 Tbsp. of juice, then cover and refrigerate the remaining rounds.Add the orange juice to a bowl with the zest. Whisk in 2 Tbsp. olive oil. Season to taste with salt and pepper.
Cut off the top stalks from 2 medium fennel bulbs. Reserve the fronds and stalks.
Cut each bulb in half vertically. Using a knife, V-slicer, or mandoline slicer, cut the fennel into slices about 1/16” thick.Transfer the sliced fennel bulbs to a medium bowl and toss with the dressing. Cover and marinate for at least 30 minutes to up to 2 hours.
Cut 1 small red onion into half-moons. Start by cutting off the stem end of the onion. Then, slice in half through the opposite end (the root). Peel off the outer layer of skin from the onion. Set the onion cut-side down on the cutting board, and slice into half-moons.
Just before serving, taste the salad. Add 1 Tbsp. red wine vinegar if you desire. Adjust the salad to taste with salt and pepper, then serve over reserved orange slices. Coarsely chop a Tbsp. of fennel fronds and sprinkle over the salad, then serve chilled.
Try serving this salad over a bed of spring salad mix! Red leaf lettuce and arugula are great choices for pairing with orange fennel salad.
If serving over lettuce, wait until after the salad is done marinating and is chilled before adding the lettuce.