Step 1Whisk together the mustard, vinegar, shallot and chives. Whisk in the olive oil until the vinaigrette is slightly thickened, about 1 minute. Season to taste with salt and pepper. Set aside.
Step 2Place the tomatoes and arugula in a large bowl and toss with the vinaigrette. Lightly sprinkle with the fleur de sel and pepper to taste.