Approx. 20 (1/2-inch thick slices supermarket French or Italian bread
10 tablespoons unsalted butter, softened
8 medium shallots, minced (about ½ cup)
3 (10-oz.) package frozen chopped spinach, thawed and squeezed dry
Salt and ground black pepper
1 cup medium-dry white wine, Vermouth
10 ounces Gruyere cheese, grated (about 2 1/2 cups)
13 large eggs
3 1/2 cups half and half
Adjust oven rack to middle position and heat oven to 225 degrees. Arrange bread in single layer on 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 4 tablespoons butter; set aside.
Heat 4 tablespoons butter in medium nonstick skillet over medium heat. Sauté 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 1/2 cup, 2 to 3 minutes; set aside.
Arrange half the buttered bread slices, buttered-side up, in single layer in dish. Sprinkle half of spinach mixture, then 1 1/4 cup grated cheese evenly over bread slices. Arrange remaining bread slices in single layer over cheese; sprinkle remaining spinach mixture and remainder of cheese evenly over bread.
Whisk eggs in medium bowl until combined; whisk in reduced wine, half-and-half, salt, and pepper to taste.
Pour egg mixture evenly over bread layers; cover surface flush with plastic wrap and refrigerate at least 1 hour or 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 bake until both edges and center are puffed and edges have pulled away slightly from sides of dish, 55-60 minutes. Cool on wire rack 5 minutes; serve with cheddar cheese sauce.
Cheddar Cheese Sauce:
Melt 5T.butter, add 6T.flour and ½ t. salt and cook. Add 1t. Worcestershire sauce, 3C. milk, and 4C. cheddar cheese