Preheat the oven to Gas Mark 4, 180°C (350°F).
To cook: Place the potatoes in a 30cm x 20cm buttered baking dish. Pour over the hot stock then dot with the butter. Season and bake for 20 minutes until almost tender.
Meanwhile, heat a little oil in a frying pan. Add the bacon lardons and cook until just crispy. Remove from the pan and set aside. Add the spinach into the same pan and allow to wilt. This will take about 2 minutes.
When the potatoes are ready remove them from the oven. Dot the cherry tomatoes on top of the potatoes and then add the spinach, bacon lardons and mozzarella. Make four indents with the back of a spoon, large enough to hold an egg. Break the eggs into each of the indents. Season the eggs then sprinkle over the grated cheese, making sure to cover the yokes 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.