Potato, spinach and tuna bake

Ingredients

• 1 kilogram potatoes, peeled, cut into 5mm slices
• 200 gram chopped baby spinach leaves
• 1 onion, finely sliced
• 40 gram butter
• 2 tablespoon plain flour
• 2 1/2 cup milk
• 1 425 gram can tuna, drained, flaked
• 1/2 cup grated parmesan, plus 1/4 cup extra
• 1 grated zest lemon