Mix the mince with the bread slices that you've soaked in water and strained, the finely chopped onions, 2 eggs and the spices. Stir well and leave it to sit in the fridge.
Next, form meatballs the size of an egg. Arrange them in an oven dish, make a nest in each of them. Pour on 1/2 of the butter, which you've melted beforehand. Bake in a preheated 428°F (220 °C) oven about 20 min.
In the meantime, boil the potatoes and mash them with the milk and a little butter. Stir well and add the final egg.
Once you take the meatballs out of the oven, stuff them with the mashed potatoes. Dissolve the tomato paste with a little water and pour it over the meatballs. Then return the oven dish to the oven again and bake briefly.
Serve the meatballs warm and sprinkled with grated cheese.