- 14 ounces tuna steak, about 2 inches thick
- 4 tablespoons extra-virgin olive oil
- 1 tablespoon fresh lemon juice, or to taste
- 1 garlic clove, crushed
- 1 tablespoon Dijon mustard
- 1 (15 ounce) can cannellini beans, drained and rinsed
- 1 small red onion, thinly sliced
- 2 red bell peppers, seeded and thinly sliced
- 1/2 cucumber
- 3 1/2 ounces fresh watercress
- Salt and fresh-ground pepper
- Lemon wedges
- Brush a ridged, cast-iron grill pan or heavy skillet (preferably cast iron) with a little oil and heat over medium heat. Season the tuna steak on both sides with coarsely ground black pepper.
- Sear the fish in the hot pan over medium-high heat 4 minutes on each side — the outside should be browned and crisscrossed with dark lines from the pan, while the inside should be light pink in the middle: Take care not to overcook the tuna, or it will become tough and dry. Remove from the pan and leave to rest while preparing the rest of the salad.
- Mix together the oil, lemon juice, garlic, and mustard in a salad bowl. Season with salt and pepper to taste and add more lemon juice, if needed. Add the cannellini beans, onion, and peppers to the bowl. Cut the cucumber lengthwise into quarters, then cut the quarters across into 1/4-inch slices. Add them to the salad bowl together with the watercress. Toss the salad together.
- Cut the tuna into slices about 1/2 inch thick. Arrange the slices on top of the salad and spoon up a little of the dressing over the fish. Serve with lemon wedges.