Preheat oven to 225°F. Arrange bread slices on a baking sheet and bake until dry, about 40 minutes, flipping halfway through. Once cool, butter one side of each slice with 2 tablespoons butter.

In a skillet over medium heat, melt 2 tablespoons butter. Sauté shallots until translucent, about 3 minutes. Add spinach, salt, and pepper, cooking until combined, about 2 minutes. Remove from heat and set aside.

Add wine to the skillet, increase heat to medium-high, and reduce to 1/4 cup, about 2-3 minutes.

Butter an 8-inch square baking dish with 1 tablespoon butter. Layer half the bread slices, buttered side up, and top with half the spinach mixture and 1/2 cup cheese. Repeat with remaining bread, spinach, and another 1/2 cup cheese.

Whisk eggs, reduced wine, half-and-half, salt, and pepper. Pour over the bread layers. Cover and refrigerate at least 1 hour or overnight.

Preheat oven to 325°F. Remove cover from strata, top with remaining cheese, and bake until puffed and golden, about 50-55 minutes.