Cut the bread into 1-inch cubes and spread them on a baking sheet. Toast in a 350°F oven for 8-10 minutes, stirring halfway through, until the edges are golden but the centers remain slightly soft. This removes excess moisture and adds structure to the pudding so it won't become mushy. While the bread toasts, finely slice the rhubarb into 1/4-inch pieces and measure out all remaining ingredients, ensuring the egg is at room temperature for a smooth custard.

In a medium bowl, whisk together the room-temperature egg, milk, cream, and sugar until well combined. Add the nutmeg and cinnamon, whisking to distribute the spices evenly throughout the mixture. The custard should be smooth and homogeneous—if you see any egg streaks, whisk a bit longer. I like to strain the custard through a fine-mesh sieve to remove any lumps, which ensures an incredibly silky texture in the finished pudding.

Butter a 9x13 inch baking dish (or similar size) with the 1.5 oz butter, using about half of it to coat the bottom and sides. Layer half of the toasted bread cubes on the bottom, then scatter the sliced rhubarb evenly over the bread. Top with the remaining bread cubes, creating an even layer. Drizzle the remaining butter over the top of the bread. This layering approach ensures the rhubarb distributes its tartness throughout the pudding rather than settling to one spot.

Pour the custard mixture from Step 2 evenly over the bread and rhubarb, making sure to cover all the bread pieces. Let the assembled pudding sit at room temperature for 20 minutes—this allows the bread to absorb the custard gradually and ensures even moisture distribution. The bread should feel slightly moist to the touch but not swimming in liquid.

Place the soaked pudding in a preheated 350°F oven and bake for 30-35 minutes, until the custard is set but the center still has a slight jiggle when you gently shake the dish—this residual carryover heat will set it completely. The top should be golden brown in spots. Remove from the oven and let rest for 5 minutes while you finish the custard sauce. I find that a bread pudding that's slightly underdone continues to cook as it cools and stays creamy, whereas overbaking makes it dry.

While the pudding bakes, prepare the custard sauce according to its instructions (the recipe indicates 0.75 yield custard). Once prepared, stir in the 0.75 tsp freshly grated orange zest and the 3 tbsp rhubarb compote, folding gently to combine. The rhubarb compote adds tartness and visual appeal, while the orange zest brightens the sauce with citrus aromatics. Taste and adjust sweetness if needed.

Spoon warm bread pudding into individual bowls or plates and drizzle generously with the orange custard sauce. Serve immediately while the pudding is still warm and the custard sauce is pourable. The contrast between the warm, custardy bread pudding and the bright orange custard creates an elegant dessert.