Preheat the oven to 425 degrees F.
Using a serrated knife, shave off the thicker parts of the crust
from the bread loaf. Cut the bread into 1-inch cubes and place in a large bowl. Spread the bread cubes in a single layer on a 13 by 18-inch rimmed baking sheet. Bake until lightly toasted, about 8 minutes. Remove from the oven and place in a large bowl. Leave the oven on.
In a skillet, melt
4 tablespoons of the butter over high heat and cook, without stirring, until it turns nut brown, about 5 minutes. Reduce the heat to medium, add the onion and a pinch of salt, and cook, stirring occasionally, until the onion
begins to brown, about 5 minutes. Add the thyme and cook for 10 seconds to release its fragrance.
Pour the onion mixture over the toasted bread. Add 3/4 cup of the Parmesan
and toss well.
In a medium bowl, whisk
together the milk, eggs, and nutmeg
. Pour the milk mixture over the bread and toss well.
Butter a 9 by 13-inch baking dish
with the remaining 1 tablespoon butter. Arrange the toasted bread evenly in the baking dish. Pour the custard evenly over the bread. Scatter the blue cheese and the remaining 2 tablespoons Parmesan over the top. Bake until the bread is golden brown and crispy on top and a thin knife blade inserted into the custard
comes out almost clean, about 30 minutes. Remove the dish from the oven and let cool for 15 minutes. Cut into individual servings and serve warm.