In a wide, shallow dish, thoroughly whisk the eggs, whole milk, heavy cream, sugar, vanilla extract, ground cinnamon, nutmeg, and fine sea salt together until uniform and completely combined.

Set a wide skillet or flat griddle over medium heat and melt 15 ml of unsalted butter until bubbly and foaming, swirling to coat the surface evenly.

Dip each slice of bread into the prepared custard, allowing it to soak for roughly 15 to 20 seconds per side so the interior absorbs moisture while retaining its structural integrity.

Place the soaked bread immediately into the hot pan without crowding. Cook undisturbed for 3 to 4 minutes until the bottom turns deep golden brown.

Flip each slice using a broad spatula and fry the reverse side for 3 to 4 minutes more, until puffed and cooked through to the middle. Wipe the skillet and repeat with remaining butter and soaked bread.