Put them in a bowl of cold water and let them soak for half an hour, pouring off the cloudy water.
Preheat the oven to 225 degrees Celsius/435 Fahrenheit top and bottom heat.
Meanwhile, bring a pot of water to a boil with plenty of salt. Cook the potatoes until al dente (not overcooked), about 10-15 minutes.
Drain them, let them boil dry for a while so that the excess liquid evaporates, then place them on a baking sheet with the garlic, cornstarch, spices (no salt!) and a nice drizzle of olive oil. Spoon over the top and put in the oven for 45 minutes.
After 15 minutes, stir (a bit vigorously) and after half an hour, add a few cubes of butter.
Take out of the oven and season with salt. Enjoy your meal!