If you don’t own a spice grinder, you can grind the fennel in a coffee grinder. Just be sure to completely clean it before and after. Substitute Kalamata olives if you cannot find Gaeta olives. Tuna loin is a delicious fatty cut often used for sushi.
Preheat the oven to 375ºF.
Season tuna with salt and pepper.
In a spice grinder, pulse the fennel seeds to a powder. Place evenly on a baking sheet. Press tuna pieces against the fennel powder, turning to cover it on all sides.
Heat a large sauté pan or skillet over medium-high heat. Add 2 tablespoons of the olive oil and sear tuna until brown on all sides, about 2 minutes on each side. (The fish should be medium-rare inside.) When finished browning, place the tuna in the oven for 3 minutes.
Remove the tuna from the oven, and cut into 1-inch slices.
Wipe the sauté pan clean and heat over medium heat; add the olive oil, olives and capers. Sauté for 1 minute, and then add the garlic and then arugula and quickly sauté until just slightly wilted. Season with salt and pepper, to taste.
Spoon on serving plates and top with sliced tuna. Garnish with tomatoes.