Grab a 9x13-inch baking dish and lightly butter or oil it. Cut your chosen bread into 1-inch cubes—this matters more than you'd think. Too small and the bread disintegrates; too large and the custard doesn't penetrate evenly. Scatter the bread cubes across the bottom of your prepared dish in a single, slightly overlapping layer. For keto versions, your bread cubes will be smaller and denser, so pack them slightly tighter. For gluten-free bread, handle it gently—GF breads are more delicate and can crumble if you're rough with them.

Pour your eggs into a large mixing bowl. If you're making the vegan version, use aquafaba instead and whisk for about 30 seconds until it's light and slightly foamy—this aerates the mixture and helps it set properly. Crack your eggs directly into the bowl and whisk them well, breaking up the yolks completely. Add your chosen milk and cream (or dairy-free alternatives), then add your sweetener, cinnamon, salt, and vanilla extract. Whisk for at least 60 seconds until everything is completely combined and the sweetener is fully dissolved. For keto versions using monk fruit or erythritol, whisk an extra 30 seconds—these sweeteners sometimes leave granules if you don't fully incorporate them.

Pour your custard mixture evenly over the bread cubes. Gently toss the bread with a silicone spatula or wooden spoon—you want every piece to get coated, but you don't want to demolish the bread. Work gently, especially with gluten-free or vegan breads. Spread everything into an even layer, pressing down just slightly so the bread sits snugly in the custard. The bread should be mostly submerged, with some pieces peeking through the top. This is where patience pays off—let it sit for 5 minutes, then give it one more gentle stir. This second toss helps the bottom pieces absorb custard too.

While your bread and custard are getting acquainted, prepare the topping. Combine your light brown sugar, granulated sugar, cinnamon, nutmeg, and cardamom (if using) in a small bowl. Stir everything together until the spices are evenly distributed—no clumps. Cut your cold butter into small pieces directly over the bowl. For vegan or dairy-free versions, cut your vegan butter the same way. Using a pastry blender, two forks, or your fingertips, work the butter into the sugar mixture until it resembles coarse breadcrumbs. You want some small pea-sized pieces and some finer crumbs—this creates textural contrast when it bakes. Cover this topping with plastic wrap and refrigerate.

Cover your French toast casserole tightly with plastic wrap or aluminum foil. Place it in the refrigerator for at least 8 hours, ideally 12 hours. This time is critical—it allows the bread to fully absorb the custard. The longer it sits, the more custardy the interior becomes. Your topping stays cold and separated in its own container. You can prep this dish up to 24 hours ahead, making it perfect for holiday mornings or when you know your schedule will be chaotic.

Preheat your oven to 350°F (175°C). Remove your French toast casserole and topping from the refrigerator about 10 minutes before baking—this helps it bake more evenly. Remove the plastic wrap and sprinkle your prepared topping evenly across the entire surface. Don't press it down; let it sit naturally on top. Bake for 45-50 minutes. You're looking for a golden-brown top and a casserole that jiggles just slightly in the center when you gently shake the pan. If your top is browning too quickly, tent it loosely with foil for the last 15 minutes. For vegan versions, you might need an extra 5-7 minutes since aquafaba takes slightly longer to set than eggs.

Remove your masterpiece from the oven and let it rest for at least 10-15 minutes. This allows the custard to set completely and makes serving cleaner. Cut into 8-12 portions (depending on appetite) and serve warm. The topping will have crisped up beautifully, creating a gorgeous contrast with the custardy interior.