Butter a 9-by-13-inch baking dish. Shingle the bread in the baking dish in two rows lengthwise.

Add the eggs, milk, brown sugar, mascarpone, vanilla, and lemon zest to a large pitcher. Using an immersion blender, blend until smooth. Pour the custard over the bread. Add the frozen blueberries over the top. Wrap and refrigerate overnight.

When ready to bake, preheat the oven to 350 degrees F.

Uncover the casserole and bake until the custard is set and the bread has crisped, 40 to 45 minutes.

Meanwhile, make the glaze by adding the juice from half of the zested lemon to the powdered sugar in a bowl, mixing until smooth. Cover and set aside until ready to serve.

Remove the French toast from the oven and drizzle over the lemon glaze. Dust the top with powdered sugar and garnish with the sliced lemons. Serve warm.