Saute chopped garlic & onions for about 2 minutes (Add optional meat). Add chopped tomatoes, tomato sauce, salt & pepper. (add finely chopped chilli pepper sparingly to taste) Simmer on low heat until sauce is thick. Slice sweet potato into thin strips
Slice zucchini lengthwise into very thin strips, add zucchini and salt to a bowl and and set aside for 10 minutes. (Salting reduces moisture content). After, blot the excess moisture with a paper towel. Then grill/fry both sides of the zucchini & sweet potato strips.
In casserole repeat layers of sauce, zucchini/sweet potato, cheese until all ingredients are used up. Cover with foil and bake 30 minutes (190°C/375°F). Uncover the foil and bake for another 30 minutes add extra cheese if desired.