Caramelize 1 cup sugar in a pan and pour it at the bottom of a cake form.
Separately, break 4 eggs in a bowl, add 2 cups milk, 3 tbsp sugar and orange zest. Beat with a mixer and pour into the cake form.
Stir the mixture well and pour it into the cake form over the 1st mixture using a ladle to help you.
Place the cake form in another tray with warm water.
Bake in a preheated 392°F (200 °C) oven for 15 min., then lower the heat to 347°F (175 °C).
Once it's ready, leave it to cool completely, place in the fridge for several hours and invert onto a platter.