8-10 slices supermarket French bread or Italian bread (6-7 ounces)
5 Tablespoons unsalted butter, softened, divided
4 medium shallots, minced (½ cup)
1 (10-ounce) package frozen chopped spinach, thawed, and squeezed dry (or substitute your farm-fresh spinach by cooking it down in a skillet until dry)
1 teaspoon salt
Pepper to taste
½ cup medium-dry white wine, such as Sauvignon Blanc
6 ounces Gruyere cheese, grated (½ cup)
6 large eggs
1¾ cups half-and-half
Adjust oven rack to middle position and heat oven to 225 degrees. Arrange bread in a single layer on a large baking sheet and bake until dry and crisp, about 40 minutes, turning slices over halfway through drying time. (Alternatively, leave slices out overnight to dry.) When cooled, butter slices on one side with 2 tablespoons butter; set aside.
Heat 2 tablespoons butter in medium nonstick skillet over medium heat. Saute shallots until fragrant and translucent, about 3 minutes; add spinach and salt and pepper to taste, and cook, stirring occasionally, until spinach and shallots are combined, about 2 minutes. Transfer to medium bowl; set aside. Add wine to skillet, increase heat to medium-high, and simmer until reduced to ¼ cup, 2 to 3 minutes; set aside.
Butter 8-inch square baking dish with remaining 1 tablespoon butter; arrange half the buttered bread slices, buttered-side up, in single layer in dish. Sprinkle half of spinach mixture, then ½ cup grated cheese evenly over bread slices. Arrange remaining bread slices in single layer over cheese; sprinkle remaining spinach mixture and another ½ cup cheese evenly over bread. Whisk eggs in medium bowl until combined; whisk in reduced wine, half-and-half, 1 teaspoon salt, and pepper to taste. Pour egg mixture evenly over bread layers; cover surface flush with plastic wrap, weigh down with a plate or bowl, and refrigerate at least 1 hour or up to overnight.
Remove dish from refrigerator and let stand at room temperature 20 minutes. Meanwhile, adjust oven rack to middle position and heat oven to 325 degrees. Uncover strata and sprinkle remaining ½ cup cheese evenly over surface; bake until both edges and center are puffed and edges have pulled away slightly from sides of dish, 50 to 55 minutes (or about 60 minutes for doubled recipe). Cool on wire rack 5 minutes and serve.