An orange-mustard vinaigrette perfectly complements mellow fennel and peppery arugula in this easy-to-make salad.
Working over a medium bowl, cut out orange segments, then squeeze juice from membranes. To a jar, add 2 tablespoons juice, vinegar, mustard, and oil; season with salt and pepper. Shake to combine.
Combine fennel and fronds, arugula, and orange segments in a bowl. Drizzle with dressing and toss to combine.
Make dressing in a small airtight jar up to a day ahead, then shake to combine and pour on salad to serve.