Put the zucchini, potato and onion into a deep bowl and sprinkle with 1/2 cup flour, salt and pepper to taste (careful! the cheese will have lots of salt don't use too much), all the grated cheese, mint, garlic, olive oil, and toss to coat all slices.
Pour this mixture into a greased (olive oil) 13 X 9" pyrex baking dish (this doesn't have to be pretty- it won't show much when baked and sliced- taste is everything!).
Press down with palms of hands to compact somewhat. Carefully pour 1/4 cup water over top.
Bake for about 1 hour and 20 minutes. (if boureki appears to be too dry during baking, you can add up to 1/4 cup more water - but careful, the zucchini and potatoes will also release liquid during baking).