Spray a large non-stick frying pan with olive oil spray and place over high heat. Add the mince and garlic and cook, stirring with a wooden spoon to break up any lumps, for 2-3 minutes or until heated through. Add the taco sauce, taco seasoning and water and cook, stirring, for 1 minute or until heated through. Remove from heat. Add the green shallot and stir to combine. Transfer to a heatproof bowl.
Wipe the frying pan with paper towel. Spray with olive oil spray and heat over medium heat. Place 1 tortilla in the pan. Sprinkle with a little mozzarella and top with one-third of the mince mixture, using the back of a spoon to evenly spread. Top with one-third of the tomato, a little mozzarella and another tortilla. Cook for 1-2 minutes or until golden underneath. Turn and cook for 2-3 minutes or until golden underneath and heated through. Transfer to a baking tray and cover with foil. Place in oven to keep warm. Repeat, in 2 more batches, with the remaining tortillas, mozzarella, mince mixture and tomato, reheating and spraying the pan with olive oil between batches.
Combine the spinach, avocado and olives in a bowl. Cut the quesadillas into quarters. Divide among plates and serve with the salad and lemon wedges.