How to Make It
Preheat the oven to 325° and lightly butter a 3-quart baking dish. On a rimmed baking sheet, spread the bread in a single layer and bake until crisp but not browned, about 10 minutes. Let the bread cool at room temperature.
In a large bowl, whisk the eggs with the half-and-half, 1 teaspoon of kosher salt and 1/2 teaspoon of pepper. Carefully fold in the toasted bread.
In a medium skillet, melt the 3 tablespoons of butter. Add the ham and cook over moderate heat, stirring occasionally, until lightly browned, about 5 minutes. Add the collard greens, onion, red pepper and a pinch each of salt and pepper. Cook, stirring occasionally, until the vegetables are softened, about 5 minutes. Remove the skillet from the heat and let cool slightly. Scrape the ham and cooked vegetables into the large bowl with the soaked bread. Fold in the grated cheese and sliced scallions. Transfer the custard to the prepared baking dish and smooth the top with a spatula. Cover with foil and refrigerate for at least 4 hours or, preferably, overnight.
Preheat the oven to 350°. Remove the strata from the refrigerator and let sit at room temperature for 1 hour. Bake, covered, for 30 minutes. Uncover and bake until the strata springs back lightly when touched and a skewer inserted in the center comes out clean, about 30 to 40 minutes more. Let rest for 5 minutes before serving.