Preheat your oven to 225°F (107°C) and arrange the crusty bread in a single layer on a baking sheet. Bake it for approximately 40 minutes until dry and crisp.

In a medium skillet, melt 2 tablespoons of butter over medium heat. Add shallots and sauté until fragrant, about 3-4 minutes. Toss in the spinach, seasoning with salt and pepper, and cook until wilted, about 2-3 minutes.

Pour the white wine into the skillet and turn the heat to medium-high. Bring to a simmer and reduce until about 1/4 cup of liquid remains, approximately 5-7 minutes. Let cool slightly.

Grease an 8-inch baking dish with butter. Layer half the bread cubes, half the spinach mixture, and 1/2 cup Gruyère cheese. Repeat the layers.

In a large bowl, whisk together the eggs, wine reduction, half-and-half, and a pinch of salt and pepper. Pour this custard mixture over the layered bread and filling. Cover tightly if chilling overnight.

Refrigerate for at least 1 hour or overnight to absorb flavors.

Preheat your oven to 325°F (163°C). Remove the cover and sprinkle remaining Gruyère cheese on top. Bake for 50-55 minutes until puffed, golden, and set.

Cool for about 5 minutes before slicing and serving. Enjoy the delightful layers with friends and family!