Grease a 9 x 13 inch baking dish (I love any of the Fiesta baking dishes!).: The warm scent of butter or oil as you grease the pan primes the bake by preventing sticking and encouraging even browning along the edges. I use a light coating that glints on the surface of the baking dish, which helps the bottom slices crisp without tearing when served. Why this matters, the grease creates a tiny barrier so the custard does not fuse to the glass or metal, making it easier to lift out perfect squares. A common mistake is using too much, which can pool and make the base greasy; use a thin, even layer and tap out any excess.
In a large bowl, whisk together the eggs, egg yolks, coconut milk (or milk), honey, vanilla, rose water, cinnamon, nutmeg and salt.: As you whisk, the mixture should smooth into a glossy pale custard, the aroma of vanilla and cinnamon rising gently. The whisking incorporates air and fully blends the honey , preventing sticky pockets, while the yolks add velvet richness that you can almost see in the sheen. This step determines the texture of the soak so getting it homogenous is essential; if it looks streaky, keep whisking until uniformly colored. Watch for overwhisking if you are hand mixing, which can create too many bubbles and a less dense set, so whisk just until smooth.
Submerge each piece of challah bread in the egg mixture, allowing the bread to sit at least a minute and soak up the eggs. Arrange the dipped slices of challah in the prepared baking dish. Pour the remaining egg mixture over the bread slices in the baking dish. Add half the raspberries in between the slices and on top.: When you press the challah into the custard, you should see the liquid pulled into the crumb, the surface darkening slightly as it soaks. The bread should feel heavy but still hold its shape, not falling apart. Laying the slices snugly in the dish helps them support one another during baking, and nesting some raspberries between slices scatters juicy pops through the interior. The remaining custard poured over ensures any air pockets fill, creating a unified set. A common error is letting the bread soak too briefly so the center remains dry, so be patient and allow at least a minute per side for thorough absorption.
In another medium bowl, combine the slivered almonds, coconut flakes and brown sugar. Add the butter and use your hands to incorporate the butter into the almonds + coconut. Sprinkle the mix over top the bread slices. Sprinkle a handful of raspberries over top.: Rubbing cold butter into the dry mix yields small clumps that toast into crisp clusters, making a satisfying top layer. You should feel the mixture come together, with the brown sugar slightly tacky and the coconut flakes and almonds coated. When sprinkled, the topping should sit visibly on the bread, promising crunch as it browns, and the extra raspberries scattered on top will caramelize at the edges, adding bright bursts against the nutty topping. One pitfall is not distributing the topping evenly, which can leave some slices soft and others overly crisp; spread it uniformly for consistent results.
Cover and place in the fridge for 1 hour or overnight. When ready to bake, preheat the oven to 375 degrees F. Bake for 45-50 minutes or until the French toast is golden and crisp. If the tops of the bread begin to brown too quickly loosely cover the French toast with foil.: Chilling lets the custard fully penetrate the challah , so the center sets creamy rather than gummy; you will notice the custard tone deepen as it rests. Preheating brings the oven to an even heat so the outside crisps while the center gently comes to temperature. During baking watch for a nutty scent and golden color on the topping, and listen for a soft crackle as the sugar and butter toast. If edges brown too fast, a loose foil tent reflects heat and prevents burning while allowing the interior to finish. A common mistake is skipping the rest, which can result in uneven texture, so plan the hour or overnight soak when possible.