In a small bowl, combine tomato, onion, vinegar and garlic. Mix well. Set aside.
In a small microwave-safe bowl, combine pesto sauce mix with water. Stir well. Microwave for 15 seconds.
Stir and microwave for 10 more seconds.
Add mayo and oil. Stir until uniform.
Spray a grill pan or skillet with nonstick spray. Place on stovetop over medium-high heat and allow to get hot.
Season both sides of the chicken breast with salt and pepper.
Place chicken breast into hot pan. Cook for about 3 minutes per side, until chicken is cooked through.
Top chicken with cheese. Cover and cook until melted, about 1 minute.
Spread pesto mayo on the bottom half of toasted bun.
Top with lettuce.
Add cheesy chicken.
Spread tomato topping on top bun.
Serve immediately, hot, assembled - or oven-faced to be closed by guest, if you want to look fancy, with fat fries.
