Puree anchovy, garlic, mustard, and caper and red wine vinegar. Slowly emulsify with olive oil. Season with salt and pepper.
For the salad:
Slice different tomato varieties 1/4-inch thick and place in a mixing bowl. Season with salt and pepper. Marinate with nicoise vinaigrette, then toss with green beans, red onions, and chives. Arrange tomatoes into a stack layered with green beans and onions and place in center of 4 serving plates. Garnish each stack with 4 slices of tuna. Season with salt and pepper and drizzle vinaigrette. Garnish with fresh chives.