Core the tomatoes and cut into wedges. Remove the top of the fennel (save some fronds for garnish) and slice the bulb very finely crosswise with a knife or on a mandoline.
Toss the tomatoes and fennel in a bowl with the olive oil, orange zest, orange juice, vinegar, salt and pepper. Garnish with 2 T chopped fennel fronds, season to taste, and serve.