Preheat oven to 375 degrees. Lay bacon in a single layer on a rimmed baking sheet and cook until crisp, about 15 to 20 minutes. Drain on paper towels. Cool and rinse the pan.
Line the baking sheet with foil. Trim the root ends of the scallions, halve lengthwise, and chop into about 1-inch pieces.
Lay out the bread on a work surface and butter 1 side of each piece. Put 4 slices of the bread, buttered side down, on the prepared baking sheet. Layer half of the cheese slices on the bread. Layer the bacon and scallions on top of the cheese and cover with the remaining cheese. Top with the remaining slices of bread buttered side up and bake until each bottom pieces of bread is golden brown, about 10 minutes. Flip the sandwiches and bake until the other side is golden brown and the cheese has melted, about 8 to 10 minutes more.