Into a saucepan, start by adding the peeled medium potatoes. Add enough water to cover them all and boil them until al dente.
Once they've cooked through, drain the water and set them aside.
Dice the onion.
Into a skillet over medium heat, add the butter and let it melt, add the onion dices and stir.
While the onions are cooking, cut the ham into dices, transfer them to the skillet, sauté a little, turn off the heat and set aside.
Grate all the potatoes and put them in a bowl, add the ham we've sautéd.
Grate the mozzarella cheese and transfer it to the bowl as well.
Add the table cream, the mustard, season with saffron, salt and black pepper, mix very well until combined.
Add a drizzle of olive oil into a glass baking dish and spread well, greasing the whole bottom.
Transfer the whole mixture to the baking dish and spread very well, leaving it as flat as possible.
Place in a preheated oven at 200°C (392°F) and bake for 30 minutes.
Take it out of the oven and, with a metal round cookie cutter, make 6 holes in the potato, and use a spoon to remove all the potato inside the cutter.
Fill each hole with an egg.
Return to the preheated oven and bake for another 30 minutes.