1. In a large pot of boiling, salted water, cook the spaghettini until just done, about 9 minutes. Drain. Rinse with cold water; drain thoroughly.
2. In a large glass or stainless-steel bowl, whisk together the vinegar, orange zest, orange juice, lemon juice, mustard, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Add 6 tablespoons of the oil slowly, whisking. Add the spaghettini, cucumbers, and mint and toss.
3. Heat a grill pan or heavy frying pan over moderately high heat. Coat the tuna with the remaining 1/2 tablespoon oil and sprinkle with 1/8 teaspoon salt and a pinch of pepper. Cook, turning once, until done to your taste, about 2 minutes per side for medium rare. Remove. Cut the tuna into 1-inch chunks and toss with the spaghettini.
Fish Alternatives: You could use salmon fillets or swordfish steaks in place of the tuna. For either, add a few minutes to the cooking time.
Wine Recommendation: Whether you make this salad with tuna or one of the alternatives listed in the box below, a dry riesling from Alsace will be splendid. Its lime, mineral, and herbal notes will brilliantly echo the salad and set off the fish.