1 tablespoon white-wine vinegar
2 tablespoons olive oil
Coarse salt and ground pepper
5 navel oranges
3 to 4 fennel bulbs (about 2 pounds total), ends trimmed, quartered lengthwise, cored, and thinly sliced, crosswise, plus 1/4 cup roughly chopped fennel fronds (optional)
In a large bowl, whisk together vinegar and oil; season with salt and pepper.
Using a sharp knife, slice off both ends of each orange. Following the curve of the fruit, cut away the peel and white pith. Halve orange from top to bottom; thinly slice crosswise. Transfer oranges, along with any juices that have accumulated on work surface, to bowl with dressing. Add fennel and, if desired, fronds. Toss to combine.
Source: Martha Stewart