Cut brioche into 1½-in thick slices. Arrange slices in a single layer on a generously buttered rimmed baking sheet.
Slowly pour the custard mixture over the brioche slices, till all the custard is absorbed.
Transfer to pie dish, and chill for 1 hour.
Heat oven to 350F. Place pie dish in middle rack of oven, and bake covered, with foil, for 10 minutes.
Remove foil and continue to bake for another 15-20 minutes, till the sides of the dish appear golden, and the custard has set (you still want a bit of a wobble).
Sprinkle with icing sugar, and dust with crushed unsalted pistachios. Enjoy with maple syrup and rhubarb compote.