Thaw tuna, if frozen. Rinse tuna; pat dry. Cut into 1-inch pieces. Place tuna in a plastic bag set in a shallow dish. For the marinade, in a small bowl stir together the lemon peel, lemon juice, cooking oil, rosemary, garlic, salt, and pepper. Pour over tuna. Close bag. Marinate in refrigerator for 2 hours, turning bag occasionally.
Meanwhile, scrub potatoes; cut any large potatoes in half. In a medium saucepan cook potatoes in a small amount of boiling water for 15 to 20 minutes or just until tender; drain. Rinse with cold water. Set aside.
Drain tuna, reserving marinade. On 4 long metal skewers, alternately thread the tuna, potatoes, and squash or zucchini, leaving about 1/4-inch space between pieces. Spray the unheated rack of a broiler pan with nonstick coating. Place kabobs on rack. Brush with marinade. Broil 4 inches from heat for 5 minutes. Turn kabobs; brush with some of the marinade. Discard remaining marinade. Broil kabobs for 3 to 4 minutes more or until tuna flakes easily when tested with a fork and squash is tender. To serve, place a tomato on the end of each kabob. If desired, serve with rice. Makes 4 servings.