In a bowl, combine all the ingredients. Season with salt and pepper. Refrigerate until ready to serve.
In a saucepan of salted boiling water, cook the quinoa for about 15 minutes, or until it tender. Drain and rinse under cold running water. Press the quinoa against the colander to remove excess water. Let cool.
Meanwhile, in a non-stick skillet over medium heat, soften the zucchini, carrot, leek and garlic in 1 tbsp of the oil. Season with salt and pepper. Transfer to a colander placed above a bowl. Press the vegetables against the colander to remove excess water. Transfer to a large bowl and refrigerate for 15 minutes, or until the vegetables are completely chilled.
Add the quinoa, egg and bread crumbs to the chilled vegetable mixture. Mix well. With your hands, shape into 4 patties of about 1/2 cup each.
In the same non-stick skillet over medium-high heat, cook the patties for 2 to 3 minutes on each side in the remaining oil (3 tbsp).
Assemble: Spread half of the tahini sauce over the bottom half of the buns, and cover with the patties. Spread the remaining sauce over the patties. Add the toppings. Close the buns and serve immediately!