Butter both sides of each piece of bread with the softened butter and arrange on 2 baking sheets. Bake, flipping halfway through, until golden brown and toasted, about 15 minutes.
Line the bottom of a 13-by-9-inch baking dish with 6 slices of toasted bread and toss together the Cheddar and mozzarella in a bowl. Sprinkle the toast evenly with about 1 cup of the cheese mixture and a third of the scallions, then sprinkle with a few grinds of the pepper. Repeat this process with another layer of 6 slices of toast, 1 cup of cheese, a third of the scallions and black pepper. Add the final pieces of toast on top.
In a large bowl, whisk together the tomato soup, milk and eggs until smooth and well-combined. Pour the tomato soup mixture over the casserole evenly so all of the toast is coated and covered. Finish off the top with the remaining cheese and scallions.
Bake the lasagna until the sides are bubbling and the top is browned in spots, about 35 minutes. Let rest for 5 minutes, then cut into squares and serve.
Store leftovers, covered, in the refrigerator. Before reheating, cut the casserole into slices. Reheat the slices in an oven or a toaster oven at 350 degrees F until the cheese is melted.
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