Place the potatoes, milk, tuna, ricotta, spinach and flour to a bowl and stir well to combine.
In a frypan, heat the olive oil over medium heat and add the onion and garlic. Sauté for 4 minutes, stirring occasionally. Add to the potatoes and mix well. Using 1/4 cup of mixture, shape the mix into sausage shapes and place into the prepared tray. Place into the refrigerator for approximately 1 hour.
Fill with oil and preheat a 3 litre deep fryer to 180°C.
In 3 separate bowls, place the flour in one, egg in one and breadcrumbs in one. Dip croquettes, 1 at a time, in egg then roll in breadcrumbs to coat.
Cook croquettes, in batches, turning, for 3 to 4 minutes or until golden. Serve immediately.