- 3 new red potatoes
- Salt and freshly ground pepper
- 4 large tomatoes
- 1 teaspoon Dijon mustard
- 2 tablespoons red-wine vinegar
- 1 tablespoon freshly squeezed lemon juice (1/2 lemon)
- 3 tablespoons olive oil
- 1 small fennel bulb, cut into 1 1/2-inch-long matchsticks, plus fronds for garnish (optional)
- 1 Kirby cucumber, seeded and cut into 1 1/2-inch-long matchsticks
- 1/2 bunch arugula (1 1/3 cups), cut into thin strips
- 4 red pearl onions, peeled and cut into thin rounds
Place potatoes in a small saucepan; cover with salted water. Bring water to a boil, and cook potatoes until fork tender, 12 to 15 minutes. Remove pan from heat, and transfer potatoes to an ice-water bath until cool. Drain potatoes, and slice into 1 1/2-inch-long matchsticks; set aside.
Slice off the tops of tomatoes; discard tops. Using a melon baller, scoop out the seeds and flesh; discard. Set the tomato "bowls" aside.
In a small bowl, whisk together mustard, vinegar, and lemon juice. Season with salt and pepper. Slowly whisk in oil. Set vinaigrette aside.
In a medium bowl, combine potatoes, fennel, cucumber, arugula, and onions. Add reserved vinaigrette; toss gently to combine. Divide salad among tomato bowls, and garnish with fennel fronds.