Step 1

Adjust oven rack to middle position and heat oven to 225°F. Arrange bread in single layer on baking sheet and bake until dry and crisp, about 40 minutes, flipping slices halfway through baking. When cooled, spread 1 side of slices with 2 tablespoons butter; set aside.

Step 2

Meanwhile, melt 2 tablespoons butter in 10-inch nonstick skillet over medium heat. Add shallots and cook until softened and translucent, about 3 minutes. Add spinach, season with salt and pepper, and cook, stirring occasionally, until spinach is warmed through, about 2 minutes. Transfer to medium bowl; set aside. Add wine to skillet, increase heat to medium-high, and simmer until reduced to 1/4 cup, 2 to 3 minutes; set aside.

Step 3

Grease an 8-inch square baking dish with remaining 1 tablespoon butter. Arrange half of bread slices, buttered side up, in single layer in dish. Sprinkle half of spinach mixture and 1/2 cup Gruyère evenly over bread slices. Repeat layering with remaining bread slices, remaining spinach mixture, and 1/2 cup Gruyère. Whisk eggs in medium bowl until combined, then whisk in half-and-half, reduced wine, and 1 teaspoon salt and season with pepper. Pour egg mixture evenly over bread layers. Wrap dish tightly in plastic wrap, pressing plastic flush to surface of strata. Weigh down strata and refrigerate for at least 1 hour or up to 24 hours.

Step 4

Heat oven to 325°F. Remove dish from refrigerator and let sit at room temperature for 20 minutes. Remove weights and plastic and sprinkle remaining 1/2 cup Gruyère evenly over top of strata. Bake until edges and center are barely puffed and edges have pulled away slightly from sides of dish, 50 to 55 minutes. Let cool on wire rack for 5 minutes. Serve.