Grate the zucchini in a colander over the sink and press to drain the water from the courgette.
In a large bowl, combine tuna, potatoes, grated carrot, zucchini, green onions, salt, cayenne pepper and black pepper and gently mix. You may need to smash the potatoes separately for the desired consistency.
Add the flour and egg and combine.
Heat olive oil in a large skillet over medium high heat. Scoop a big tablespoon of batter for each cake. Use 2 tablespoons to make the desired shape (please check this video for instructions). Otherwhise you can flatten the batter with a spatula.
Cook and flip until all sides are nicely golden brown, about 2 minutes each side.