Preparation:

Season tuna with salt and pepper and grill for 4 to 5 minutes per side (less time if you prefer rarer fish).

While tuna is cooking, wash and drain arugula and reserve. Remove core from the fennel bulb and slice fennel. Set aside in a large bowl. 
Cut oranges into sixths; remove pith, skin, and seeds. Add to arugula.

Make vinaigrette by whisking the olive oil slowly into the vinegar. Add salt and pepper to taste.

When tuna is finished cooking, remove from the grill and allow to rest a couple of minutes. While fish is resting, dress salad and toss.

Serve salad on four individual plates. Slice tuna and place warm tuna atop each salad.