Preheat oven to 180°C. Line a baking tray with non-stick baking paper. Cook the potato in a saucepan of salted boiling water for 12 minutes or until tender. Drain and return to the pan. Add 1 tablespoon of the milk. Use a potato masher to mash until smooth. Transfer to a large bowl and set aside for 5 minutes to cool.
Add the spinach, tuna, cheddar, shallot, lemon rind and 1 egg to the potato. Season with salt and pepper and stir to combine.
Whisk the remaining milk and eggs in a bowl. Place the flour and breadcrumbs on separate plates. Shape 2 tablespoonfuls of potato mixture into a pattie. Repeat with remaining potato mixture to make 16 patties. Dip 1 pattie in the flour to coat. Shake off excess flour. Dip in egg mixture then in breadcrumbs to coat. Place the pattie onto the prepared tray. Repeat with the remaining patties, flour, egg mixture and breadcrumbs.
Spray patties with oil. Bake for 20 minutes or until golden. Serve the patties with tomato sauce.