Lightly grease a 9x13-inch (23x33 cm) baking dish or casserole dish.
Cut the bread into 2.5 cm (1-inch) cubes and spread them evenly in the prepared dish.
In a saucepan over medium heat, whisk together the plant-based milk, rum (if you are using it), vegan butter, starch, coconut sugar, cinnamon, nutmeg, vanilla extract, lemon zest, and salt until smooth and the butter has fully melted.
Pour the mixture evenly over the bread cubes, pressing firmly with a spatula to ensure the bread absorbs the liquid. If preparing in advance, cover and refrigerate for up to 12 hours.
Preheat the oven to 350°F (175°C). Cover the casserole with aluminum foil and bake for 35 minutes.
Remove the foil and continue to bake for an additional 15-18 minutes until the top is firm and lightly browned.
Remove from the oven and let it cool for 10 minutes before serving. Top with vegan whipped cream, fresh berries, powdered sugar, and maple syrup as desired.