Preheat a griddle to 350°F, or medium-high heat. Position a cooling rack inside a rimmed baking pan and set it nearby. The rack will allow surplus custard to drain from the soaked bread.
Place the eggs, whole milk, orange juice, vanilla extract, granulated sugar, lemon zest, cinnamon, nutmeg, and salt in a large mixing bowl. Whisk thoroughly until the custard is smooth and evenly blended.
Working with one slice at a time, immerse both sides of the pumpkin brioche in the prepared custard, making certain the bread is evenly saturated.
Arrange each coated slice on the prepared cooling rack. Continue until all the brioche has been dipped, then distribute any remaining custard over the slices.
Leave the bread on the rack for 5–10 minutes. This resting period permits excess liquid to drain while allowing the interior of each slice to develop a soft, custard-like texture.
Transfer the soaked brioche slices to the heated griddle. Cook the first side for approximately 3–5 minutes, or until evenly golden brown.
Turn each slice and continue cooking for another 3–5 minutes. Thick pieces may require additional time so that the centers become fully warmed while remaining tender.
Place the finished French toast on serving plates and present it warm with a pat of cool butter, a drizzle of chilled syrup, or other desired toppings.