1. Place tuna steaks in plastic resealable bag. Add 3 tablespoons of the teriyaki; seal and let marinate at room temperature for 30 minutes.
2. Cut tomatoes in half; squeeze out seeds. Chop into 1/2-inch pieces. Spread out on paper towels; sprinkle with salt. Cover with a layer of paper towels and let stand 15 minutes.
3. Pat tomatoes dry and place in medium-size bowl. Stir in remaining 2 tablespoons of the teriyaki, onion, cilantro, lime juice and tequila, if using. Let stand at room temperature until ready to serve.
4. Meanwhile, heat gas grill to medium-high or prepare a charcoal grill with medium-hot coals. Remove tuna from bag; discard marinade. Place on grill and cook about 3 minutes per side. Serve with tomato salsa on the side.