Preheat the oven to 200°C, gas mark 6. Pack the spinach into a saucepan, cover with a lid and cook gently for 2-3 minutes until wilted. Drain in a colander and spoon half into the base of a shallow baking dish. Arrange half the onion and potato slices on top, then layer with the remaining spinach, onion and potatoes.
Crumble the cheese into a saucepan and add the garlic and stock. Cook gently, stirring frequently for a few minutes until the cheese has melted into the sauce. Blend the cornflour in a small bowl with two tbsp water and add to the pan. Heat until bubbling and thickened. Pour over the potatoes and bake for about 40 minutes until the potatoes are crisp and the sauce is bubbling. Serve with a leafy salad.
Cooking for a crowd? This is rich and cheesy so only small portions are required. For really hearty appetites or when cooking for larger numbers, cook some sausages in the oven alongside the bake.