How to Make It
Cook the sausage in a large skillet over medium-high heat, breaking it into pieces, until browned, 4 to 5 minutes. Reduce heat to medium and add as much kale as will fit. Cook, tossing and adding more kale as there is room, until wilted but still bright green, 1 to 3 minutes.
Place the bread in a buttered 3-quart or 9-by- 13-inch baking dish and top with the cooked sausage and kale.
Whisk the eggs in a large bowl. Whisk in the milk, mustard, 1 teaspoon salt, and ½ teaspoon pepper. Stir in the cheese. Pour over the bread mixture; cover and refrigerate for 1 hour and up to overnight.
Heat oven to 350° F. (If the strata is refrigerated overnight, remove from the refrigerator 40 minutes before baking.) Bake until golden brown and set around the edges but still slightly wobbly in the center, 40 to 45 minutes. Let cool for 15 minutes before cutting. Serve warm or at room temperature.