Preheat the oven to Gas Mark 4, 180°C (350°F).
To cook: Arrange the thinly sliced potatoes in a buttered 30cm x 20cm baking dish. Pour over the hot vegetable stock and dot with butter. Season and bake for 20 minutes until almost tender.
Meanwhile, heat a drizzle of oil in a large frying pan over a medium heat. Fry the bacon lardons until crisp and golden. Transfer to a plate and set aside. In the same pan, sauté the spinach for 1–2 minutes until just wilted.
When the potatoes are ready, remove them from the oven. Scatter the cherry tomatoes on top of the potatoes, then add the spinach, bacon lardons, and mozzarella.
Make four indents with the back of a spoon, large enough to hold an egg. Crack the eggs into each indentation. Season the eggs, then sprinkle over the grated cheese, making sure to cover the yolks to prevent them from overcooking
Place the dish in the oven and bake until the eggs are just set, 10 to 15 minutes.
To serve: divide into four portions or bring the dish to the table and let everyone help themselves.