Step 1

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

Step 2

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.

Step 3

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

Step 4

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

Step 5

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.

Step 6

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.