To prepare salad, cook pasta according to package directions, omitting salt and fat. Drain and rinse under cold water. Drain; place in a large bowl.
Heat a large grill pan coated with cooking spray over high heat. Sprinkle tuna with 1/8 teaspoon salt. Add tuna; cook 5 minutes on each side or until desired degree of doneness. Remove from pan; cool slightly. Cut tuna into 1-inch pieces. Add tuna, cucumber, and next 5 ingredients (cucumber through tomatoes) to pasta.
To prepare dressing, combine lemon juice and remaining ingredients, stirring with a whisk. Drizzle over salad, and toss gently to coat.