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1
Preheat oven to 425u00b0F. Toast bread on a rimmed baking sheet until dry, 8-10 minutes.
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2
Meanwhile, heat a large skillet over medium-high. Add sausage and cook, breaking into pieces with a spatula, until browned, about 7 minutes.
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3
Butter a 3-quart baking dish. Arrange 6 bread slices in dish in one layer, covering bottom of pan completely and cutting slices to fit if needed. Tear remaining slice into smaller pieces to fill in any gaps.
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4
Using a slotted spoon, transfer sausage to baking dish, scattering over bread.
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5
Heat skillet over medium and add white and pale green scallion parts. Cook, stirring, until softened and just starting to take on color, about 3 minutes. Add spinach, stir to combine, and cook until warmed through, 1-2 minutes more. Top sausage with spinach mixture, then top spinach mixture with cheese.
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6
Whisk eggs, milk, yogurt, mustard powder, salt, pepper, and nutmeg in a large bowl. Gently pour over casserole. Cover and chill at least 4 hours or up to overnight.
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7
Uncover dish and transfer to a cold oven. Heat oven to 375u00b0F and bake casserole until eggs are puffed and cheese is browned, about 40 minutes.
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8
Let casserole cool several minutes. Top with dark green scallion parts.
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9
Casserole can be assembled 1 day ahead. Tightly cover and chill.
