Preparation:

Peel and dice onion. Set aside. Dice zucchini and set aside. 

Heat a sauté pan on medium heat. Add olive oil. Add onion and cook until translucent. Then add zucchini and cook until slightly soft. Season with salt and pepper. Set aside to cool. 

Beat the egg and mix with ground turkey. Add cooled onion and zucchini. Form four burger patties. 
Heat a sauté pan to a medium-high heat, add vegetable oil, and when pan is sufficiently hot, add burgers. Cook 4 minutes on each side or until cooked through. (Burgers can be grilled or baked.) Place one slice mozzarella on each burger and heat until melted. Place burger on bun and garnish with lettuce and tomato slices.