Serve your favorite salad ingredients stuffed inside a ripe, juicy tomato.
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.