This dish will need to be refrigerated over night, so you will want to make this ahead of time.
8 to 10 slices day-old French bread (you can toast bread)
5 tablespoon unsalted butter, softened
4 medium shallots, minced (about 1/2 cup)
10-oz. package frozen chopped spinach, thawed, drained, and squeezed dry
1/2 cup dry white wine
6 oz. Gruyère cheese, grated
6 large eggs
1 3/4 cups half-and-half
Butter the bread slices on 1 side with 2 Tbsp of the butter and set aside.
Heat 2 Tbsp of the butter in a medium nonstick skillet over medium heat. Sauté shallots until 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 a medium bowl. Add wine to skillet, increase heat to medium-high, and simmer until reduced to 1/4 cup, 2 to 3 minutes. Set aside.
Butter an 8-inch square baking dish with remaining 1 tbsp of butter; arrange half of the buttered bread slices, buttered-side up, in a single layer in the dish.
Sprinkle half of the spinach mixture, then 1/2 cup grated cheese evenly over bread slices. Arrange remaining bread slices in a single layer over the cheese, then sprinkle remaining spinach mixture and another 1/2 cup cheese evenly over the bread.
Whisk eggs in a medium bowl, then whisk in reduced wine, half-and-half, 1 tsp salt, and pepper to taste.
Pour egg mixture over the bread layers. Cover the surface with plastic wrap, and weigh down a ziplock of rice, lentils, beans or sugar. Whatever you have on hand. Refrigerate overnight.
Remove dish from refrigerator and let stand at room temperature for about 20 minutes.
Heat oven to 325 degrees. Uncover the strata and sprinkle remaining 1/2 cup of cheese evenly over the top. Bake until both edges and center are puffed and edges have pulled slightly away from sides of dish, about 50 to 55 minutes
Cool on a rack for 5 to 10 minutes, then cut into squares and serve.