Preheat the oven to 350ºF (180ºC) and generously butter a 9x13-inch (23x33-cm) baking dish.
Zest the lemon directly into a medium-sized pot. Add the whipping cream, whole milk, and raisins. Place the pot over medium-low heat and heat until steamy.
1 medium lemon, 1 ¼ cup 35% whipping cream, 1 cup whole milk, ½ cup raisins
While the custard base is warming, juice the zested lemon directly into a mixing bowl. Add the eggs, egg yolks, and sugar. Whisk well to combine and set aside.
1 medium lemon, 3 large eggs, 2 large egg yolks, ⅔ cup sugar
Butter both sides of each slice of bread and cut each one in half diagonally. Arrange the buttered bread halves in the baking dish so they’re slightly standing and overlapping. Set aside.
½ cup salted butter, 10 slices bread
Scoop ¼ to ½ cup of the hot custard (without raisins) from the pot. Slowly dribble the hot liquid into the egg and sugar mixture while whisking constantly to prevent scrambling.
Once all of the hot cream is whisked into the eggs, pour the tempered egg mixture back into the pot of remaining custard. Cook the custard over medium-low heat until slightly thickened, about 7 to 9 minutes, or to 170º F (77ºC) whichever comes first. Transfer the raisin custard mixture to a heatproof vessel suitable for holding with your hands (optional) and stir in the vanilla extract, cinnamon, nutmeg, and salt.
Spoon the custard and raisins over the arranged bread slices in the baking dish and let soak for 15 minutes. Cover the baking dish with foil and bake for 30 to 35 minutes. Remove the foil and continue to bake for another 10 to 15 minutes or until the custard is wobbly in the middle but not wet. Cool the dish on a wire rack for 15 minutes before serving warm with a drizzle of heavy cream.